Here’s a fun fact: You probably get way less light exposure during a normal work day than you would if you were out camping.
“Sure,” you say. “That’s no surprise. At home, I have walls around me to block the sun. If I’m camping, I presumably have fewer walls.”
“You don’t understand,” I say, leaning in. “You get way, way less light exposure.”
I’m basing this off a famous circadian experiment from Ken Wright’s group at the University of Colorado Boulder, in which they compared the light people get in modern electrical lighting environments with the natural light they get while camping.
It’s not a 1x or 2x difference when you go from modern light exposure to camping light exposure. It’s a 13x difference:
Thirteen times more light exposure during the day! And this in the winter! It’s nuts.
Imagine turning your current daily light exposure down by a factor of 13, or to 8% of its current brightness. Two things would probably be true. First, it would be hard to see, so you’d bump into things. Second, and most important from a circadian perspective, it would be hard for your brain to tell the difference between day and night.
After all, the signal telling your brain that it’s day (light) would be just a tiny fraction of what it was before. It’d be like turning a faucet down to just a thin drizzle. You can tell it’s on if you look for it, but it’s an easy thing to miss.
In a sense, we’ve already done this with the shift from natural lighting to indoor, artificial lighting. We’ve given up the firehose of light that is sunlight exposure in favor of a much muted signal from our indoor lights and devices.
If you look at the lighting figure above from Stothard et al., it’s ridiculously easy to see where day starts and stops in the camping conditions (black line). But the picture is muddled for modern electric light (gray line): Day seems to start fairly clearly, but where does it end? There’s this blunted peak in light exposure during the day, and a long, ambiguous tail of light exposure stretching out into the night hours. There’s not really a clear day/night divide.
This matters for our health. There’s a notion in circadian rhythms science of your circadian “amplitude.” Roughly, you can think of amplitude as a measure of how confident your body’s clock is about the time it thinks it is. Give your circadian clock a clear day/night signal, and this will boost the amplitude. Keep it on a constantly changing, dim-light-round-the-clock kind of schedule and the amplitude goes down.
In other blog posts, I’ve talked about your phase response curve, which tells you which direction (earlier, later) light will push you when you get exposed to it. But you can also think of the amplitude response curve, which tells you whether your amplitude will go up or down if you get light at a certain time. Generally speaking, the amplitude response curves in our models tell you to go outside right smack in the middle of the day if you want to boost your amplitude as efficiently as possible.
So this Thanksgiving, get some outdoor light. Sure, yeah, get some exercise while you’re out there if you want. But simply being outside and in the light is a good thing: It’s building stronger, more robust rhythms in your brain. And if you happen to fall asleep hard after eating a big meal– well, part of it might be that your circadian clock’s a little more confident that day is over and it’s time to snooze.
For almost a century and a half, it was thought that the mammalian retina had just two types of photoreceptors: rods and cones. That assumption was not proven to be false until studies in the late 1990s proved the existence of a third kind of mammalian photoreceptor that differed greatly from rods and cones. These new types of photoreceptors were retinal ganglion cells (RGCs) that were intrinsically photosensitive (ipRGCs)— or in other words, naturally sensitive to light.
Though the official evidence to determine that ipRGCs actually existed did not come until much later, this third class of photoreceptor had already been hypothesized in 1927, nearly seven decades earlier, by a graduate student named Clyde Keeler. During one of his studies, he examined the behavior of mice that lacked nearly all rod and cone function as a result of severe retinal degradation, which left them functionally blind. Keeler noticed that despite the lack of rods and cones, the mice still had a very strong and significant pupillary constriction in response to light, and he determined that this response must have been the result of some third photoreceptor in the retina. The lack of concrete evidence for a whole new photoreceptor at the time resulted in this pupillary response being explained away by other scientists. However, in 1999, Russell Foster and his team would revisit Keeler’s work armed with a new host of tools.
Foster et al. worked with mice, much like Keeler did, but in their case, the mice being observed were genetically engineered to not have any rods or cones. Yet regardless of their missing rods and cones, the rats still displayed strong pupillary light reflexes and were even able to shift their circadian rhythms with shifting light exposure schedules. With these studies complete, the presence of a third photoreceptor was almost confirmed, but some still weren’t convinced because nobody had found another light-sensitive molecule (opsin) in the mammalian retina yet.
The discovery of melanopsin in the photosensitive skin cells of frogs occurred in 1998, and in the following four years studies determined that the very same opsin was being expressed in a small percent of RGCs in both mouse and human retinas. This discovery allowed scientists to easily mark ipRGCs and confirm their existence, which finally put to rest the debate of whether or not there was a third class of photoreceptor.
So they exist, but what do they do?
IpRGCs differ greatly from rods and cones when it comes to how they work. Their main function in the body is to signal the intensity of ambient light levels (irradiance) to the brain. These signals are largely used for non-image-forming visual reflexes that are subconscious, such as pupillary constriction, neuroendocrine regulation, and synchronizing daily circadian physiological rhythms to environmental light. This means that the way ipRGCs respond to light by themselves is also quite different from rods and cones.
As mentioned before, these photoreceptors use melanopsin as their photopigment. and that makes them more responsive to light at around 480nm (blue light). In the graph below, you can see that this wavelength is significantly different from the best wavelengths for stimulating rods and cones (panel b).
Although ipRGCs function as photoreceptors themselves, it was found that they additionally receive synaptic input from the circuits of rods and cones. This means that ipRGCs have both an intrinsic light response coming from melanopsin and an extrinsic one that is mediated by synaptic input from rods and cones. The light response caused by melanopsin is markedly different from that of rods and cones: ipRGCs have both an intrinsic and sluggish light response as well as an extrinsic, rod/cone driven, rapid photoresponse. There is an ongoing debate about the relative significance of this extrinsic synaptic input and the role rods and cones play in determining our circadian rhythms.
A recent case study:
In a recent research article by Mouland et al., their team assessed whether the effective light intensity registered by melanopsin (blue light ~480nm) was a more important determinant of circadian impacts than that of cones under realistic contrast scenarios. The ability to determine melanopsin’s contribution to circadian light responses comes from the evolution of a color science technique which is referred to with multiple names, such as receptor silent substitution or metamerism in colorimetry. Metamerism occurs when two colors appear to match under a specific lighting condition but have different underlying spectra.
This technique allows for the stimulation of specific photoreceptor classes, like ipRGCs. Mouland and colleagues quantified the circadian impacts of different photoreceptors by recording electrophysiological activity from the suprachiasmatic nucleus (SCN) of anaesthetised mice while they were presented with movies. The movies were either high or low contrast and had varying irradiances specialized for the distinct photoreceptor classes.
During the experiment, the energy response recorded from the SCN closely tracked with melanopsin-driven signaling across all conditions. In general, steps in melanopic irradiance were determined to be the most significant factor accounting for light-induced changes in SCN activity. The only cone-directed lighting patterns with significant impacts on SCN activity were low contrast movie conditions. Basically, this study suggests that cones do have an impact on the circadian signal going to the SCN in some conditions, but the influence of melanopsin on the circadian signal is far more consistent.
This blog post was written by Arcascope’s intern, Ali Abdalla. Thanks, Ali!
This post used Webvision as a major resource. Thanks to Dustin Graham and Kwoon Wong for the excellent review.
I recently got some blackout curtains for my bedroom. This was pretty long overdue: about thirty feet from my bedroom window is a cheerfully bright, energy-efficient street lamp, which—while great when I’m taking the dog out for a nighttime stroll—is the photic equivalent of somebody standing in my azaleas and playing “Seventy-Six Trombones” while I’m trying to sleep.
I’ve definitely started sleeping longer since I’ve gotten them. But I’ve also noticed that they’ve made it so I need to be even more careful about my other sources of light at night. The reason? They don’t just block my light at night. They also block light in the morning.
I’m thinking about this because it’s almost the end of daylight savings time, and, once again, there’s talk of making it permanent. As a quick guide: Daylight savings time (DST) is the one where the clocks move forward (so it’s lighter at night), while standard time is the one where the clock moves back (darker at night). The “Sunshine Protection Act”, introduced by Florida senator Marco Rubio, encourages states to observe a permanent version of DST, with the argument being that lots of good things could come out of just chilling it with the time change.
Permanent daylight savings time means not having to change the clocks, and not having to experience that gnarly “lose an hour” in the Spring. It means no confusion about how many hours offset we are from the time in the U.K. and no struggling to remember if you should say EDT or EST when you’re trying to coordinate a Zoom meeting across time zones. As a programmer, I’m generally in favor of anything that makes the totally miserable experience of interacting with dates and times in code even marginally easier.
But it also means—and I’m talking about permanent daylight savings time here—lots and lots of dark in the mornings.
This is bad. It’s bad because light at night is fundamentally different from light in the mornings, because our bodies are fundamentally different at night than they are in the mornings.
Light in the morning does a couple things, but one of the most important ones is that it “advances” our circadian rhythms. It tells our internal clock that night is over and it’s time to get a move on. It makes it easier to fall asleep at night.
And if you get a lot of light in the morning, it eventually advances you to the point where… it stops advancing you. You enter the part of your daily rhythm where light delays your clock. A sort-of “slow down, what’s the rush” period of your internal rhythm that starts in the mid-afternoon for most people and continues into the early morning.
And that slowdown period is the problem. Because while light in the morning is hitting you in the advance region, which you eventually get advanced out of, light at night is hitting you in the delay region, which is like a temporal sand trap. When you get light exposure in the delay region, your clock gets slowed down, which means you spend more time in the delay region. Which means you don’t feel tired as quickly, which means you get more light, which means you spend even more time in the delay region. It’s a feedback loop that spins out of control. It might be the reason that night owls exist.
So if we adopt permanent DST, we’re adopting a schedule where we get more light during the hours most people call night, and much less light in the hours we consider morning. We’re setting ourselves up to fall into the delay region sand trap: More light in the night, making us stay up later and get delayed, and far less light in the AM hours to counteract it.
This is what tanked permanent DST the first time we tried it. I’m not sure why this doesn’t always get brought up as the very first point against permanent DST, but we’ve totally done it before. In 1973, anywhere from 57-73% of people supported staying on DST during the winter. So they did it, in January of 1974. By the time February and March rolled around, only 19-30% of people still thought it was a good idea, while 43% said it was actively bad.
What changed? People experienced what happens to your body when you have to kick off your day in the dark of night. They drove to work and caught the bus to school, while the sun waited to rise until 8:00 am. They didn’t like it, and rolled the decision back before the next winter came around.
You might say, “well, time is a fake idea. Who says you have to start your day before 8:00 am?” This is a fair point. We could, societally, shift the normal times we do things to match whatever schedule we wanted. In China, where the entire country is on the same time zone, places like Kashgar (in the far west) have shifted their normal operating hours to reflect the fact that the sun might not come up until 10 am.
But it’s a lot tougher to change social standards of when school and work “should” start in every town in the country than it is to pass a bill changing the time that appears on your phone. Which is why we shouldn’t do it: Permanent DST will put us on schedule where our traditional social standards for when things should happen are at odds with our biology, sabotaging our sleep and circadian health.
If we want to stop the whiplash of changing the clocks twice a year, why not do permanent standard time? I’m in favor of this. It reduces confusion the same way permanent DST does, but without the corresponding damage to our internal rhythms. Sure, it might mean that 9:00 pm is dark, even in the summer. But darkness at the right times is a healthy thing. And from a safety perspective, there are lots of street lamps and other sources of light at night these days that are very good at their jobs.
Which brings it back to me and my blinds: I’ve needed to be more careful about my other sources of light at night lately because my blackout curtains mean I don’t get woken up by the sun. That’s not a big problem: I can wake up in the dark and yank them open myself, like one of the townsfolk in the first song in Beauty and the Beast.
But if I get too much light at night from non-streetlamp sources, like watching Succession on my computer or looking at Succession memes on my phone, my ability to wake up in the dark in the morning is going to be less reliable, jeopardizing my exposure to that vital morning light. And I’m lucky that there’s even morning light to get: with permanent DST, I could be hopping on my first calls of the day while the sky is still black outside.
My point is that social pressures already make it hard for us to get the darkness we need at night (let’s face it, screens are fun) and the light we need in the morning. We shouldn’t make it harder for ourselves with a change to a system that’s already failed once. Permanent daylight savings time is a no-go. Permanent standard time? Call me.
Lately, I’ve been watching clips from the Olympics, getting misty-eyed when the athletes hug at the end, and then stalking the winners on social media: a normal Saturday night in 2021. I have a shirt that looks like that, I think as I scroll through photos of them lifting approximately three times my body weight. We’re not so different, you and I, I think as they hit a five-inch target from a football field away.
Coming from the East Coast, this was a three-hour shift west. To adjust to California time, I needed to delay my circadian clock. One catch, though: my flight out was extremely early in the morning. That meant, like it or not, I was going to advance myself as I set out on the journey.
Let’s back up a little. We talk about directions your internal clock can shift as advances or delays. Think of advancing as hustling your clock along, making your circadian rhythms more like those of people in time zones east of you. Delaying, on the other hand, is like a temporary slowdown for your clock, making your rhythms more like people living to your west. Light at different times of the day advances or delays you, depending on your clock’s state when you’re exposed to it.