Numlock News: August 12, 2022 • Sleep Deprivation, Sleep Restriction, Sleep Regularity
By Olivia Walch
Today your guest host is my friend Olivia Walch. She is the writer of the new book about the mathematics of redistricting Political Geometry, the founder and CEO of sleep science company Arcascope and the creator of comics such as You Can Try Again and Imogen Quest.
Rise and shine, Numlock readers! I’m bringing you an all-sleep newsletter this fine Friday morning. Get out your herbal tea and your comically overlarge sleeping cap, and settle in for a snoozefest, starting with:
You Can Go a Long Time Without Sleep
Does not sleeping for an extended period of time make you permanently psychotic? That’s what researchers in 1968 set out to study when they kept four subjects awake for 205 hours. And by “kept awake” I mean “threatened to withhold compensation from everyone if anyone fell asleep during the trial.” Enter: loud music, bright lights, and dunking heads into ice buckets as the subjects did everything in their power to keep from snoozing and losing out on earning $400 for nine days without sleep. While the subjects hallucinated, complained about each other and were generally pretty miserable throughout the study, the researchers did a follow-up a year later and decided that three of the four subjects weren’t really that different than they were before the study. (One of the four couldn’t be followed up with because he was on the run from the FBI, but since he was on the run before the trial began, they decided that wasn’t the fault of the whole not sleeping thing.) In the words of the researchers, “We [conclude] that sleep deprivation per se is unable to produce psychopathological reactions which extend beyond the period of sleep deprivation.” Wow, seems like maybe sleep isn’t that important after all!
Of course you should sleep. And it’s not just for your brain health either: If you’re regularly getting less than seven hours of sleep a night, you’re 2.94 times more likely to get a cold than those sleeping eight hours or more. Researchers uncovered this by having 153 healthy volunteers record their sleep every day over a two-week baseline period, then taking them into a lab and giving them nose drops containing a rhinovirus. The ones reporting higher sleep duration during the baseline period were significantly less likely to develop a cold than those with short sleep durations. Curiously, how well rested the subjects felt during the baseline period was not associated with how likely they were to get sick. So even if you’re feeling pretty decently rested up, you may want to steer clear of scientists carrying rhinovirus nose drops.
On the Topic of Unreliable Self-Perception of Sleep
Being kept on a chronic sleep restriction schedule of four or six hours of sleep per night for 14 days makes you about as bad at reacting to things as you’d be after a double all-nighter. Here’s what I mean by that: Chronic sleep restriction is what it sounds like — namely, researchers don’t let you sleep more than N hours a night. A very common performance test to give people you’ve been sleep-restricting is the psychomotor vigilance test (PVT). In the PVT, a number pops up on a screen at random times, and you’ve got half a second to respond to it. Take more than half a second, and you’re said to have “lapsed.” People who are sleep deprived lapse a lot, and the number of lapses you lapse after two weeks on four or six hours of sleep a night looks a lot like the lapses you’d lapse if you’ve been up for 48 hours. You basically get worse and worse at the test with every passing day of sleep restriction. But your subjective sleepiness — your perception of how sleepy you are — doesn’t worsen at nearly the same rate. Instead, so long as you’re getting at least four hours a night, you don’t seem to recognize that your performance is tanking. Quoth the authors: “[O]nce sleep restriction is chronic, subjects either cannot reliably introspect with regard to their actual sleepiness levels, or [d]o not experience a sense of sleepiness anywhere near the levels found for total sleep deprivation.” Cool, cool. I can do more than six hours of sleep a night. That’s all I have to worry about, right?
Sleep duration is probably the most popular sleep-related number, but evidence is growing that sleep regularity may be just as important in some contexts. Sleep regularity can be defined a bunch of different ways, but it essentially boils down to how consistent your sleep habits are. You can have a high sleep duration and a low sleep regularity by simply sleeping eight hours a night, but starting your sleep at wildly different times each night. One reason you might not want to do that: Older individuals with irregular bedtimes were more than twice as likely to develop cardiovascular disease over five years as those with more regular bedtimes. Another study looking at the same cohort found that every one-hour increase in the standard deviation of sleep duration was associated with a 27 percent increased odds of developing metabolic syndrome. It’s not totally clear what a causal mechanism for this could be, but the primary suspect is that irregular sleep disrupts your body’s internal circadian clock, which governs sleep and a whole host of other things as well: immune response, metabolism, grip strength, you name it. This work, along with the increasing body of research on “social jet lag,” suggests you probably want to avoid disrupting your body’s clock.
Wait, What’s Social Jet Lag?
Defined roughly as the difference between the midpoint of your sleep on work days and the midpoint of your sleep on your days off, social jet lag is jet lag without travel. Sleep in and stay up late on the weekend, and then feel miserable on Monday morning? That’s social jet lag. Thought of another way, social jet lag captures the tug-of-war between your biological clock, which gets set by your light exposure, and the social constraints imposed by school, work, and just generally hanging out with people. Time zone boundaries are the perfect place to study this tug-of-war, as you can have two groups of people subjected to about the same light schedule (i.e. the sun) but different social schedules (“workin’ 9-to-5”). The picture isn’t pretty for those on the western edge of time zones: Researchers looking at time zone boundaries found that having an extra hour of light at night associates with about 19 minutes fewer of sleep per night, as well as worsened health and economic measures.
Turn the Lights Down
You’ve probably heard about blue light at night messing up your sleep and circadian rhythms. You may not have heard that light doesn’t mess up everyone in the same way. Researchers at Monash University found a greater than 50-fold difference in sensitivity to evening light across individuals. They got this result by having people spit into tubes under different lighting conditions in the hours before bed. The spit was to check for melatonin, a hormone your body naturally produces during the biological night, which gets suppressed in the presence of light. For some people, 10 lux of light (really dim) was all it took to suppress melatonin production by half, while for other people it took 400 lux to get the same effect. One takeaway is that two people living in the same space might be experiencing radically different effects from the same bedside table lamp. In other news, I once worried that the bedside table lamp in my guest room was too bright for Walter’s circadian clock, only to learn the next morning that he’d slept with the overhead light on the entire night. A very, very bright overhead light, which I specifically chose to be bright, because…
Light Isn’t the Enemy
Not all the time, at least. One reason your body’s clock wants you to avoid light at night is so it has an easier time telling night from day. Another way to make your day/night signal clearer? Get loads more light during the day. People who are camping get more than four times as much light during the day as people who are living in electrical lighting environments (4487 vs 979 lux). Another fun thing that happens while camping? The distribution of chronotypes — think: morning larks and night owls — shrinks so that there are way fewer night owls. It might be that night owls are mostly a thing because of electric lighting, and that your personal light sensitivity may heavily influence whether you’re a night owl or not.
Thanks for having me, Numlock readers! Take care, sleep well, and for more of my writing on sleep and circadian rhythms, check out my company’s blog.
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