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Episode 13: Blue Light and Sleep
11 Plays5 years ago
We examine a paper that finds sleep disruption from using tablet computers (as compared to reading a book in dim light). How much should we be worried about the effects of screens on a good night's sleep?
There's good reason to believe that blue light is the main culprit -- recently discovered receptors in the eye that respond to blue light directly connect to brain areas implicated in sleep regulation. We lay out the case....
Papers:
"Evening use of light-emitting eReaders...."
"Melanopsin: photoreceptors, physiology and potential"
Recommended
Transcript
Introduction to Cognation Podcast
00:00:06
Speaker
This is Cognation, the podcast about cognitive psychology, neuroscience, philosophy, technology, the future of the human experience and other stuff we like. It's hosted by me, Joe Hardy, and by me, Rolf Nelson. Welcome to the show.
Impact of Light Exposure on Sleep
00:00:24
Speaker
All right, today on this episode of Cognation, we're going to be talking about the effects of light before you go to bed and whether that
00:00:36
Speaker
can negatively impact your sleep. And in particular, talking about the effects of using an e-reader or an iPad or other device that emits light directly, reading off of that versus reading from a book, like a regular paper book. It gets into a lot of different areas of interest for us, understanding how vision works. It's also a bit of a technology topic, and I think it's something that's of practical interest
00:01:07
Speaker
to most of our listeners, hopefully, because we all read in some capacity or another, and I think a lot of us probably use e-readers as well. That's going to be what we're talking about. The paper that we're going to be basing the conversation off of is, evening use of light-emitting e-readers negatively affects sleep, circadian rhythm, and next morning alertness.
00:01:32
Speaker
Just a little bit of background and then we'll try to describe the paper. So a lot of people might be familiar with the fact that we have a natural circadian rhythm. So our body naturally regulates things over the course of about 24 hours. And this is something that's really common throughout evolution.
Understanding Circadian Rhythms
00:01:52
Speaker
There was an interesting study a few years ago. There's only one guy who did this. It was a French guy named Michael Sifry.
00:01:59
Speaker
trying to figure out what human beings natural circadian rhythm was, decided to get rid of all light cues. He went in a cave and stayed there for 127 days straight, and he woke up when he felt like it, turned on the light when he felt like it, and eventually he kind of settled into a daily cycle, and his daily cycle turned out to be slightly longer than 24 hours.
00:02:24
Speaker
So he figured that a daily cycle for a human is about 24 hours and 30 minutes long, which is strange. It's a little longer than a regular day. Since then, people have done more precise calculations of about what a circadian day is. And it's about 24 hours and 18 minutes is the biggest estimate.
00:02:43
Speaker
So what it seems to depend on is an internal sense of time. So we have an area of our brain called the suprachiasmatic nucleus, which is in the thalamus of the brain right there in the middle. That seems to keep about a 24-hour clock or maybe just a little longer than 24 hours. Then in order to sync this up with the external world, we have
00:03:07
Speaker
different kinds of cues that are called zeitgebers. That's a German term, just means a time giver. This is something that will sync it up with the actual world that we're living in. Light seems to function as a strong zeitgeber. In other words, your 24-hour cycle is internal, but it needs to regulate itself by getting light in the morning and then darkness at night.
00:03:35
Speaker
A lot of things go along with the circadian rhythm, so you have a lower temperature at night and you fall asleep more easily at night after you've had a little bit of darkness and all kinds of metabolic processes are tied with this.
Modern Lighting and Sleep Disruption
00:03:52
Speaker
One thing that is obviously prevalent in the modern world is light that's a little bit out of sync with the rising and setting of the sun. We have light bulbs, we have
00:04:05
Speaker
light sources from all over the place. We don't necessarily go to bed right when half an hour after the sun sets, we can stay up as late as we want and maybe our schedule of sleeping changes from night to night. We may stay up till 11 or 12 one night and then stay up until 10 the next night and it doesn't have that regular cycle that the sun has. There's been a lot of talk recently about how electronic devices in particular can
00:04:35
Speaker
affect these kinds of circadian rhythms. And especially if you're staying up late reading on an iPad or whatever device that maybe this is actually disrupting your daily cycle and it's giving you cues that are throwing off your circadian rhythm. Yeah, that's right. And one of the things that is different about
00:04:58
Speaker
e-readers or these types of screens from the light that you might receive from just reflected off of a book, for example, is the distribution of the spectrum of the light is considerably shorter wavelength when you're reading one of these e-readers. And I think many of us may be familiar with the idea of using the night mode on an iPad, for example, where it basically just turns down
00:05:26
Speaker
the short wavelength light, which from a perceptual standpoint makes it look yellower and less blue, basically. So that the overall screen is just emitting less short wavelength light. And it seems that there's a connection between especially short wavelength light and the regulation of these circadian rhythms. And there's a bunch of interesting biology behind that that's come out over the past
00:05:55
Speaker
a little less than 20 years. But we'll get into
Harvard Study on E-Readers and Sleep
00:05:58
Speaker
that a little bit later. But I think it makes sense to dive into the paper itself and talk about the specific study because it's pretty, pretty cool, pretty interesting. The authors are Anne-Marie Chang, Daniel Eichbach, Jean Duffy and Charles Zeisler. And I think I probably butchered a couple of those things. I apologize to the authors. I don't know them personally. They are, it seems like mostly out of Boston at Harvard and bringing them women's
00:06:24
Speaker
hospital and working in one of these sleep labs where they do these really intensive inpatient designs. And the inpatient design, we talked a little bit about in some of the other pods, but it's kind of cool and pretty intense when you talk about a sleep study like this. Yeah, so basic idea. So they had they had people in the lab for 14 days straight. I believe this is day and night. Right. So they're just in the lab. They don't. They're not going anywhere.
00:06:55
Speaker
They're hanging out in the lab. Their whole environment is completely controlled. And a lot of these studies are like this. They'll keep people in the hospital, in the lab for weeks at a time. And they've got all kinds of equipment. They've got good recording equipment. They're taking blood samples for melatonin levels. They're giving some questionnaires for how sleepy they are.
00:07:19
Speaker
and they also are doing some brainwave recordings. They're doing some EEG to record different levels of brainwaves. That's right. The idea is they have two different conditions. In the two different conditions are both reading conditions and the idea is that you read for a considerable amount of time before you go to bed in this study. Every subject is reading every night, but the groups are randomized in terms of
00:07:49
Speaker
whether they're first reading off of an e-reader, in this case, an iPad, or if they're reading an actual book. And so they do this five nights in a row. They'll read five nights in a row. So group A might start with the iPad. And so every night for five nights in a row, they'll read for four hours. So from six, I think they read from 6 p.m. to 10 p.m. They get one little break in there. But other than that, they're reading for a pretty extended amount of time.
00:08:18
Speaker
Right. In the first three hours, they're sitting in a chair reading with like the iPad or the book on a stand at a set distance from their eyes. And then the last hour they sit in bed. And so it kind of is meant to be a realistic, if somewhat extreme version of what it would be like to read a bunch before you go to bed. And so, you know, group A reads for five nights on the iPad and then group B is reading for five nights
00:08:48
Speaker
on the book, and then they reverse. So then the group that was reading the iPad reads the book, and the group that was reading the book reads the iPad. How many subjects were there? There were 12 subjects. 12 subjects, yeah. And I believe it was evenly distributed, women and men. Yeah, so six male, six female. And yeah, exactly, it was a crossover design, so everyone did both conditions.
00:09:13
Speaker
basically the idea was that they were especially interested in what was happening like towards like the fourth and fifth nights and even the night after you were in each of these conditions. And so they were looking at all these different measures really more oriented towards the tail end of when you're in one of the reading conditions. Okay. So let's just describe the way that they, well, I'll just read what they give for the results.
Melatonin and Sleep Timing
00:09:44
Speaker
We found that, compared with reading a printed book in reflected light, reading an LE e-book, so this is just an iPad, in the hours before bedtime, decreased subjective sleepiness, decreased EEG delta and theta activity, and suppressed the late evening rise of pineal melatonin secretion during the time that the book was being read. We also found that, compared with reading a printed book,
00:10:08
Speaker
Reading an LEA e-book in the hours before bedtime lengthened sleep latency, so it took longer to fall asleep, delayed the phase of endogenous circadian pacemaker that drives the timing of daily rhythms of melatonin secretion, sleep propensity, and REM sleep propensity, and impaired morning alertness. So on all of these different measures, they basically found an impairment for the
00:10:34
Speaker
e-book readers or the iPad versus just reading on a paper book. So, you know, basically they found kind of what they were looking for. So this is concrete evidence that if you're doing a lot of reading on a tablet in bed, it can actually hurt the sleep that you're getting. Exactly. So when people were reading on the iPad, it took them longer to get to sleep.
00:11:00
Speaker
But interestingly, they didn't sleep less. They slept the same amount of time within the eight hour window. It just took a little longer to a little longer to fall asleep and they woke up and they were sleepier when they woke up as well. So also sleepier when they woke up. Yeah. So it's like interesting because it really does seem to be melatonin does seem to be really related to
00:11:25
Speaker
the phase of when you fall asleep. If you're taking melatonin, the idea, you know, as a supplement, for example, the idea is that you should take it to mark the time when you want to fall asleep, rather than it's not like the same thing as like one of these sleep aids that you would take that's a drug where you would
00:11:44
Speaker
like knock you out for a period of eight hours or something like that. Right. And I think a lot of people think of melatonin as being kind of a sedative that they could take a melatonin instead of an ambient or something and help them fall asleep. But it's really a way of kind of setting this circadian rhythm at the right time. Right. So I don't know. I don't have a strong opinion about that literature because I haven't really looked into it.
00:12:09
Speaker
Theoretically, it wouldn't even make sense to just take a bunch of it and try to really knock yourself out because it doesn't work that way. It's more about trying to time it. Here, the idea is that the timing of the release of melatonin is actually delayed as well. Consistent with the behavioral pattern of it takes a little longer to fall asleep, they're seeing that the melatonin release is delayed. We'll talk more specifically about the way that
00:12:39
Speaker
the way that light affects melatonin production or it directly affects what's going on in the brain. In our second paper, we did mention that it seems to be driven more by bluish light than by other parts of the spectrum. One other thing that they did in this study is they gave a readout of what kind of light an iPad is giving off. So this is if you're not setting it on that nighttime mode. And you do see this huge spike
00:13:09
Speaker
So human vision goes roughly from about 400 nanometers to a little over 700 nanometers. And blue light is in that lower range, around the 400 to 500 range. And you see a big spike in that 400 to
Biology of Blue Light and Sleep
00:13:25
Speaker
500 range. So it really is outputting more bluish light than, say, broad spectrum like sunlight.
00:13:34
Speaker
Yeah, and the broad-spectrum light that they were using in the lab there, when measured, reflected off of the book, was peaking at about 612 nanometers versus for a peak of about 450 nanometers for the iPad. And the idea here is that there are different types of photosensitive molecules in the eye.
00:14:02
Speaker
The cell types that help you see during the day are called cones, and there are basically three different types of photosensitive molecules that are associated with three different cone types, approximately. Now, there's a lot of variability there, and that's a whole different topic. We won't get into that. Collar vision, which is a fascinating topic. We won't get into that
00:14:31
Speaker
today because I will definitely go off the rails if we get too deep into that. For the sake of simplicity, there are three different cone types and there's also something called the rods and the rods help you see at night basically. Rods have sensitivity that's a little bit more short wavelength than the peak of the overall cone spectrum. They're closer to the short wavelength sensitive cones.
00:15:00
Speaker
And they help you see at night. So that's why if you want to see at night and you want to turn on a light that helps you see but doesn't knock out your night vision, you use that red light. So you might see that in like certain applications where there might be like, you know, in a submarine or something like that, you might have a red light that people are using to see so that they don't knock out their their night vision or their their rod vision.
00:15:30
Speaker
But the idea there is that the sensitivity of these different types of cells is in different parts of the spectrum. But now the cool thing is that there's a new type of, well, it's not new, but it's new to us as scientists, a new type of cell that's sometimes referred to as a melanopic retinal ganglion cell or an intrinsically photosensitive retinal ganglion cell.
00:15:58
Speaker
And the ganglion cells basically are cells that connect up with the rods and cones a little later down the stream of processing, a little closer to the brain in terms of processing. And we didn't think that these cells were necessarily, that these cells were directly sensitive to light, but rather getting signals from the rods and cones. But now we know that there are some, these IPRRGC cells. It's a mouthful, it's a tough, yeah.
00:16:26
Speaker
Yeah, it's a lot to say. But let's just say, you know, melanopic makes sense, right? Because they're making this connection between melatonin and these cells. The idea is that there's some direct connection between this circadian rhythm system and these melanopic cells in the eye. So there are cells that are sensitive to light and they're sensitive to light at the short wavelengths. And it seems like they're not really
00:16:55
Speaker
involved that much in vision, although maybe a little bit. They're mostly not involved in vision, but they may actually be involved crucially in the regulation of sleep. Now, I think what's interesting about this is that a lot of this is new, and people had an idea about blue light being
00:17:14
Speaker
something that's particularly bad for sleep, but a lot of this wasn't really known in much detail and it wasn't till about, it was about 10 years ago when the actual physiological basis of this in humans was discovered. So they knew about this, so they had some idea about this in mouse models, but
00:17:33
Speaker
in humans, we had no idea that we had specifically photosensitive retinal ganglion cells, that it wasn't just rods and cones. I think this is the motivation for this iPad study is to specifically point out that, okay, we've got iPads that are giving off relatively more blue light, and we think that that's what's responsible for
Critique of Study and Light Effects
00:17:59
Speaker
Uh, this disruption in sleep that you see all over the place and it fits with this model of these photosensitive retinal ganglion cells now. OK, so we've gone over the basics of this study. I wonder if there are any comments. So what did you? What did you think of this study, Joe?
00:18:17
Speaker
Interesting from a lot of different levels. So the fact that they saw with the e-reader that people's sleep was a little disrupted is cool and interesting. It's a small study. So with this number of subjects, you know, I don't think you get a PNAS paper, which is what the journal of this was
00:18:35
Speaker
published in Proceedings of the National Academy of Sciences, if it weren't for this connection to these, you know, these new cell, this new cell type that we're interested in now. So in other words, just the behavioral result in and of itself, which is really all they're showing, well, they have the EEG results here, but it's fundamentally a behavioral result is interesting. But it's really the it's the connection to this melatonin system that is that is that is the most interesting part. And so they have to make an argument that
00:19:05
Speaker
It's the short wavelength light in the iPad that that doesn't that has the effect for that for that connection to make sense. However, they didn't actually measure that. Right. So, for example, like a better study that would give you more insight into that would be to like read with the iPad in regular mode and then read with the iPad and.
00:19:27
Speaker
Nighttime mode. Yeah, that would be the most clear thing. Now I should say that, you know, they did a nice job with this study and it did show a new result. And I think it's important. And some of our criticisms are picky, but I agree with that. I think what you really want to know is if you're getting the same amount of light
00:19:46
Speaker
and it's just of a different spectrum, that it's really specifically blue light that's the issue, and not just the overall volume of light, especially if we're making this connection to these newly discovered cells. Yeah, because there's the overall amount of light that is reaching the eye from the e-book in this study is something like 50 times more. Yeah, it's massively more, and this is something that probably doesn't appear in the headlines of this article when it appears in popular press.
00:20:16
Speaker
They do a measurement of the amount of light that's affecting different parts of the eye. So they have six different kinds of light, photopic lux, cyanopic lux, et cetera. And the amount that comes off of the print book, so this is just lit by a dim light in the room, for example, for the photopic lux is 0.91.
00:20:41
Speaker
for the e-book or the iPad, it is 31.73, so at least 30 times as much light. So they set the iPad on the highest light setting possible, and then that's compared to a very dimly lit room. Now, I mean, if you think about it that way, you might have some objection to this and say, well, yeah, if I turned it all the way up, but usually when I'm reading in bed, I don't have it turned all the way up.
00:21:11
Speaker
probably at about its dimmest setting by the time you're in bed reading, right? Right, exactly. Exactly, yeah. But I'm pretty convinced about the overall effect of short wavelength light. I think this particular study in and of itself wouldn't stand on its own. This same group had another study, actually even a few years earlier, that more directly looked at
00:21:40
Speaker
monochromatic light, so like light of just one wavelength and exposure to that light, you know, either short wavelength light or long wavelength light. Yeah, I sort of, I see this, yeah, and the researchers are, they've certainly done a number of other good studies and I think you have to give them
00:21:59
Speaker
And I think what they're going for here is just trying to demonstrate that this actually exists in the first place. And the first thing you do in experiments is sort of throw everything at it and show the effect first. And then you can be more subtle about it. Right, exactly. So I think some of the more
00:22:14
Speaker
less headlining papers will get more into the specifics and the wavelength sensitivity of this effect and how much of it is driven by these special cells versus, you know, we also know that the rods and cones themselves are also involved in circadian rhythms. So circadian rhythms can be regulated by any of these cell types depending on, you know, for example, if you did a study with
00:22:43
Speaker
in other animals where you might knock out one or the other of these opsins or photosensitive molecules, you always get some photosensitive light-based regulation of day and night cycles as long as you have any of those light-sensitive molecules. Any of them can be involved and probably all of them are involved, but there may be a special role of this melanopic photo pigment.
00:23:13
Speaker
Which is cool, you know, so, I mean, getting into exactly, you know, what the role is and how much, you know, short wavelength life versus long wavelength, you know, all that's going to be super interesting. But I think in general, you probably as you get, you know, just from a practical standpoint, I think it does how effective the nighttime mode is, is a question mark. But it's it's certainly like something worth, worth trying. And then I would also say.
00:23:37
Speaker
In general, you don't want a lot of light before you go to bed.
Advice on Reducing Nighttime Light
00:23:40
Speaker
Yeah, I think this fits with people's common sense on this. I think, you know, everybody's heard this from a lot of sources. Don't stare at your phone a lot before bed. Lots of bright light before bed is going to make it hard to go to sleep. I think most people have experienced something like this. It was just a verification of that.
00:24:01
Speaker
And I will say, I mean, you know, when you do this, when you do the study, right, like this, the effect sizes are relatively modest. These are pretty small effects. They're talking about about a 10 minute lengthening of how long it took people to go to sleep. So this is after reading for four hours on a bright iPad or a dim book. And there's a 10 minute difference between the two.
00:24:27
Speaker
If you look at the amount of melatonin that's in their system, they actually converge about an hour or so into sleep. In other words, the difference of the effect is really mostly at the beginning of sleep and then it sort of straightens out and it's relatively, it's relatively the same. Yeah, exactly. And I mean, I think that's, that was one of my thoughts as well, reading this paper, which is why are, you know,
00:24:55
Speaker
This is, this is a little bit of a controversial topic. You know, uh, I think that usually the most interesting things are, you know, that's kind of the nature of it. And why is it controversial? Well, one of the reasons why it's controversial is it's not a huge effect because it was a huge effect. You just know you would experience it easy and obvious. Yeah. Yeah. Every person could experience it for themselves. Like for example, the effect of light when you're trying to sleep, like everyone knows that it's like,
00:25:21
Speaker
easier to fall asleep if you turn the light off, if you're a sighted person. So that's a thing. So we don't need to run a ton of studies to show that. But this is a little smaller effect. So you do need to run some careful studies. Yeah. And I remember when I was trying to get all my sleep hygiene
00:25:44
Speaker
correctly done and sleep hygiene is just kind of the term for making sure that you're relaxing before bedtime and always sleeping at the same time at night, all of these different things. There was a time when I would wear blue blocking glasses and besides looking kind of ridiculous, they didn't seem to be that effective when I tried them out. When did you wear them? So I would put them on about eight in the evening and then wear them for a couple hours before bed. If I went to bed at like 11 o'clock,
00:26:14
Speaker
I'd wear them for a couple hours. Were they the blue blockers? They were the blue blockers. And I tried to get ones that match the... Because at the time I thought, okay, well, you've got these photosensitive retinal ganglion cells that respond to this particular region of light. So I thought, okay, I'm gonna try to get ones that filter out all the light from that area, and that should be the most effective. So that's what I did. And I thought, well, okay,
00:26:43
Speaker
I don't notice a huge difference here. So I think there's so many different knobs and dials for how you fall asleep and what makes for a good night of sleep. And so many people have problems with sleep. Some of these are big problems and some of these are small problems. I think blue light is a, is a contributor, but it is not everything. I think people would be well off using their blue light settings on
00:27:12
Speaker
their iPhone if they're looking at their iPhone at night. But it's not, you know, there are lots of different things that affect sleep. Yeah, no, absolutely. No. So maybe one last comment to that before we go to break.
00:27:27
Speaker
We've heard from some people that listening to podcasts can be a good way to fall asleep. And we just want you to know that we're not offended if you're listening. Yeah, even if you've fallen asleep already, that's totally fine. That's totally cool. We're not going to implant any subliminal messages or anything, we promise. Let's take a break.
00:27:56
Speaker
Okay, we're back. We're back.
New Retinal Cells and Their Effects
00:28:01
Speaker
So in the first part of the show, we talked about some interesting research and results indicating that reading an e-reader before going to bed like an iPad might disrupt your sleep more than reading just a regular book. And one of the interesting side points related to that is that there is some new evidence that we have discovered recently
00:28:26
Speaker
relatively recently that there are cells in the eye that are sensitive to light that are not necessarily predominantly used in vision. We used to think there were two basic types of photosensitive cells in the eye, the rods and the cones, and we now know that there are other cells that are sensitive to light that are so-called photosensitive retinal ganglion cells or intrinsically photosensitive
00:28:55
Speaker
retinal ganglion cells. And these cells seem to be involved in a bunch of interesting stuff like sleep and wakefulness cycles, circadian rhythms, but also learning and memory, pupillary constriction,
00:29:13
Speaker
and even potentially mood. So there's some interesting findings that have come out over the past 20 years or so since these cells have been discovered. And it's an interesting and engaging area of new research. And so we want to talk a little bit more about this and some of the findings in the neurophysiology of this topic.
00:29:36
Speaker
The paper we're using to guide this bit of discussion, and we'll put a link to this on the site, is called Melanopsin, Photoreceptors, Physiology, and Potential. It's in the journal Current Dependent in Physiology, and it is by Paluma, Gilhooley, Jaganov, Hankins, Hughes, and Pearson. This just appeared last year in 2018. Just to get a sense of things, I'm going to read a bit of
00:30:05
Speaker
the abstract, which I think conveys some of the interest in this kind of stuff and how new it is. The discovery of melanopsin expressing photosensitive retinal ganglion cells has led to a fundamental change in our understanding of retinal light detection.
00:30:24
Speaker
PRGCs, and they get into some abbreviations here, perform a broad range of non-visual functions, most notably mediating circadian entrainment to the environmental light-dark cycle. However, over the last two decades, it's become clear that the melanopsin system is far more complex than first realized, influencing a wide range of physiology and behavior, including pupillary constriction, light aversion, sleep, learning and memory, and even mood.
00:30:54
Speaker
OK, so they talk about. All of these different things that seem to be regulated and it's a little bit of a complex system. So just to again to break down this term. So. So they're called photosensitive. Retinal ganglion cells. And photosensitive just means that it's sensitive to light that it's responsive to light. Just as rods or cones are.
00:31:24
Speaker
And then retinal just means it's in the retina at the back of the eye. Ganglion cells is a type of cell that's in the back of the eye. Now, something that people may not realize is that there are actually a number of different cells that work in our eye, not just rods and cones and retinal ganglion cells. So there are a couple layers that are happening that processes information early on in the eye. So light travels into the
00:31:53
Speaker
back of the eye first, so it goes through a number of these layers, and then it gets picked up by rods or cones, and then it gets passed on to amacrine cells, bipolar cells, and then ganglion cells. And then ganglion cells are the ones that travel through the optic nerve and go into the brain. So again, you can think of this kind of like wiring that
00:32:22
Speaker
rods and cones are the first ones to pick up information and now as it turns out these ganglion cells and then it passes through these other layers. It's really just filtering out the right kind of information and then passing it out to the optic nerve. That's right. Another interesting point related to this wiring is that the rods and cones,
00:32:50
Speaker
Photosensitive cells that are super important for vision actually are behind the ganglion cells in the way that the packing structure is in the eye. So in other words, the light that enters the eye actually goes through the retinal ganglion cells, the amacrine cells, the bipolar cells, the horizontal cells before it reaches the rods and cones.
00:33:16
Speaker
However- Yeah, this is totally counterintuitive. Yeah, we always talk about it in terms of the processing as neuroscientists and psychologists, we always talk about it in terms of the processing pathway, starting with the rods and cones. So for us, we always think about rods and cones as being primary, being first, because that's where the information flow starts for vision for the most part, overwhelmingly so. So the light is detected by rods and cones,
00:33:46
Speaker
And then that information is passed through neural signaling to the other cells. But actually the light first has to pass through the ganglion cells before it reaches the rosin cones. These cells are mostly transparent. The light passes right through. However, there are some cells now that we know.
00:34:06
Speaker
where the light is being absorbed and that absorption is impacting the processing that's happening. So this light being absorbed by this melanopsin is actually having an effect in the brain, mostly not related to perceptual vision, but mostly related to these other effects.
00:34:29
Speaker
Yeah, so just as a side note, the reason why rods and cones seem to be in the back of the eye. So you could design a visual system that worked the other way. They could be the first things to receive light. And this is actually the way that it works in some other creatures. I think octopuses work this way. But the reason why in people and most other animals we have them in the back of the eye so that they can be refreshed quickly by these
00:34:57
Speaker
photoreceptive pigments, so things like melanopsin so that they can get a steady supply of these and respond quickly to them, which seems to be more important than getting the light first before it passes through other layers. Yeah, light moves fast. Yeah. Yeah, exactly. I mean, these cells were discovered in kind of an interesting way.
00:35:27
Speaker
what people were doing was they were knocking out rods and cones in mice. So they're basically just genetically modifying these mice and knocking out these rods and cones. And guess what? They were still responding to light in terms of their circadian rhythms. So their circadian rhythms were still responding to the light, even though they didn't have rods and cones. And this made people think, hey, what's going on here?
00:35:55
Speaker
and started to look for these other cell types. Yeah, and as it turns out, it's not every ganglion cell that's responsive to light. So it's about 1% of these ganglion cells. So most ganglion cells are totally unresponsive to light, but a few of them use melanopsin and pass that signal along.
00:36:26
Speaker
And through the research that's been going on, and especially as pointed out in this paper, there's a variety of different kinds of these ganglion cells that respond to light. So they count about five different kinds of these cells. They look different. And they also seem to connect to different parts of the brain. So they
00:36:50
Speaker
They may be responsive to more than just sleep cycles. We're concerned mostly about sleep cycles here because that's kind of what got us into it. But they also seem to connect to areas of the brain that are active in light responses, so a pupillary light response. And in other animals at least, they seem to be responsive for light aversion. So if you knock out
00:37:20
Speaker
If you knock out these retinal ganglion cells in mice, you get different effects for light aversion. Then they also seem to be responsive for mood, too, at least in mice. I don't know how they measure mood in mice. Right. Yeah. It's a little hard to know what they're thinking. I guess they probably look more, you know, sullen, sullen. Just a little down. Poor little guys.
00:37:49
Speaker
But, you know, we know that people use have used this blue light, you know, therapy for mood stuff for a long time. You know, I know that we had this even when when I was a kid, this like, you know, like sort of more short wavelength light that we like my dad, I think, was using for trying to enhance his mood, basically.
00:38:14
Speaker
I don't know. Again, it's one of those things that it doesn't work so well that that it's that everyone wants to do it. So it's and it's not so immediately obvious that it that it works super well. But yeah, we're really selling it here, aren't we? Yeah. I mean, it's like, yeah, it's one of those things that if it was if it worked so awesome that you would just do it. But, you know, it does seem to there does seem to be some evidence for an impact of this. Again, is the idea that there's this short wavelength light
00:38:44
Speaker
when you're exposed to this high intensity short wavelength light, it might increase your alertness. And that also might be related to an improved performance on attention tasks. And then also maybe correspondingly some learning and memory effects as well. But basically just an arousal, it's really an arousal effect, right? It seems to be, but you get these projections that go to all these different parts of the brain and it's a little,
00:39:14
Speaker
It's a little tough to know exactly what it's all doing. I mean, this research is still in its relatively early stages, so it's hard to map out exactly where everything's going and then what exactly it's affecting and then how that translates into actual experienced results. Right. No, exactly. I mean, I think because the hypothesis would be, hey, look, if this short wavelength light is the negative and the positive, right?
00:39:43
Speaker
you don't want that short wavelength light before you go to bed because it's arousing and it triggers. Sorry. You don't want this short wavelength light before you go to bed because it's arousing and it inhibits the triggering of melatonin, but maybe you want it when you need to be alert. So maybe you actually should seek that out
00:40:11
Speaker
Yeah, in situations where you need to remember something or, you know, you need to be highly aroused, for example. Yeah, it can help cause alertness, right? Right. Yeah, I mean, it's an interesting thing. I mean, from that perspective, of course, you know, too much short wavelength light can be dangerous. So we don't want to go crazy there.
Color of Light and Sleep Promotion
00:40:32
Speaker
You know, the short wavelength is, you know, corresponds to a higher frequency and higher frequency has higher energy. So, you know, the short wavelength light
00:40:41
Speaker
is also more responsible for damage to the eye. But we're talking about now at very, very high light levels. And that's why you have, that's why your eye is slightly yellow in terms of the cornea and the crystalline lens is precisely that, to filter out that short wavelength light so that there's less damage to the eye. There's one of the more interesting things that
00:41:10
Speaker
I found here as some recent studies that indicate that it's possible that green light may be sleep promoting. Now, I wouldn't say go out and look at a whole lot of green light just before you go to bed, because it's still light. But there were some studies in mice just a couple of years ago that find these wavelength differences. And they found not only that blue light
00:41:40
Speaker
kept mice up, but that greenish light actually hastened the sleep onset. So light of about 470 nanometers, or that peaked at 470 nanometers, which is blue light, got them all excited, and elevated corticosterones, but green light, which is 530 nanometers, just a little up there,
00:42:05
Speaker
got them to go to sleep faster. So maybe this is a direction that we might see some research on in the coming years too, is that differentiating these kinds of light. I have no idea how this stuff would actually translate to people, because these are my studies and not everything translates perfectly, but it could be a possible thing to keep an eye out for.
00:42:27
Speaker
No, absolutely. I mean, it's super interesting area of research. I mean, the light has so much effect on our psyche, right? In so many ways, you know, from, you know, this, you know, activation, just overall activation, whether you're sleepy or alert, you know, your mood, I mean, just think about the effects of sunlight, you know, being able to go outside and experience sunlight versus on a cloudy day, how much better you feel?
Emotional Impact of Light
00:42:56
Speaker
And I think there's just a lot of interesting research to be done still on exactly what are the, for example, wavelength dependencies of these effects. And then what are the underlying mechanisms that are mediating these effects? I mean, there's some interesting, I'd say most of this research in terms of the effects of these
00:43:20
Speaker
photosensitive retinal ganglion cells versus, you know, rhodopsin versus, you know, versus photopsin. You know, I'd say it's mostly directional at this point, rather than, you know, super compelling as being like, this is the mechanism. This is what you want to do if you want this effect. But there's it's very clear. There's interesting stuff going on there. Well, and I think you make a really good point about being affected mood wise by different kinds of light.
00:43:50
Speaker
I think the discovery that you've got these sub-cortical pathways, in other words, we think of the main pathway of vision as going, so it goes from our eyes to the thalamus in the middle of the brain and then goes to the back of our head to the cortex. So the part that we think of as our brain and the part that does most of the cognition, that's what we think of as the main pathway and the discovery of these
00:44:18
Speaker
ganglion cells that go towards deeper structures of the brain and may affect these things like mood directly
00:44:26
Speaker
I think is a really interesting, I mean, it's really interesting in that area that a lot of these kinds of effects may be entirely subconscious and also sort of accompany other effects of how vision is working. So besides detecting patterns and detecting what color it is, we also may be getting these direct emotional responses to different kinds of light
00:44:54
Speaker
Yeah, absolutely. I mean, just two other quick points of a couple other papers that I saw on this topic. One is a paper from Brian Wandel, which actually argued that these photoreceptive retinal ganglion cells can actually be involved in image detection or that there is like some
00:45:17
Speaker
Pattern analysis. Yeah, exactly. You can actually see something with these cells in a way, or that there is evidence that they are involved in actual vision. This is what makes it complicated. The brain is just... Yeah, the brain tends to use all the information that's there, right? I mean, if there's a general rule of thumb that if
00:45:41
Speaker
If there's information there, the brain is probably using it somehow. It's just incredibly good at taking advantage of the available information. So it does seem to be involved in vision, probably in the peripheral part, the outside part of your field of view. So it's probably not a huge important effect, but it's just interesting that it does seem to have an impact. I'd be interested in knowing
00:46:11
Speaker
evolutionarily where this comes along. All of a sudden we have more cells in our eyes that are responding to light than we did before. All of a sudden we know about these. Is this something that predates rods and cones or is it something that has been tacked on? What did this melanopsin system come from? I'm super interested in hearing more about that as well because
00:46:37
Speaker
you know, what we learned in graduate school was rhodopsin, which is the, I mean, the basic opsin is like, opsin is like a photosensitive molecule, right? This thing called rhodopsin, which is the thing that's in rods and slightly modified photopsin in the cones is, you know, so highly, highly preserved across species. There aren't a lot of molecules that have this,
00:47:03
Speaker
uh, characteristic of being sensitive to light down to the level of just one photon. That's, that's super interesting thing about, about rhodopsin, right? Is that literally one photon of light
00:47:17
Speaker
can trigger a response that can't be detected in the cell. I mean, that's something you would actually see this one photon of light, but, you know, that this can be activated by just one photon. Yeah, it's that sensitive. Yeah, that's sensitive. So, you know, and it's highly preserved across species. So I'd be curious to know where in the evolutionary tree melanopsin falls, you know, compared to rhodopsin photopsin. And I'm sure that people are I'm sure a lot of people are thinking about that. I'd be curious to to hear how that how that shakes out.
00:47:46
Speaker
Yeah, what's funny, I mean, what's what's particularly interesting here is the idea that, you know, when you learn this stuff at first, everybody learns as you know, in early on in elementary school, you've got three kinds of cones, you know, you learn, you've got red, green and blue cones. And, you know, we refer to them as short, medium and long wavelength cones. But that's one of the first things you learn. And you figure, okay, we know everything there is to know about how color vision works. And I just learned all there is to know. But then you find out, wait a second, we've got
00:48:16
Speaker
It's more complicated than that, and we really don't know everything that's going on in the eye.
Conclusion and Recommendations
00:48:23
Speaker
It's just that progression of science and how you can't take for granted that you know everything that's going on. No, absolutely. Absolutely. What else don't we know? Lots of stuff that we think we know that we don't. Cool. I think this is a super interesting topic.
00:48:41
Speaker
And I'm glad that we had a chance to talk about it. And it's some practical applications here. I'm definitely not going to, I'm going to try to reduce my blue light exposure before I go to bed, for sure. Well, I certainly await future research in this area. I'll be watching this. Cool. Well, yeah, great. And I think it was probably a good place to wrap it up. And if you guys have comments, we'd love to hear them. You can tweet at me, JL Hardy PhD.
00:49:10
Speaker
at NationCog as well. You can tweet at me, R-O-F-L Nelson, and I will not respond because I never check Twitter because it rots your brain. But we really do want to hear what you have to say. We appreciate some feedback and thank you for listening.