Sleep and Body Rhythms

Frank S. Robinson
4 min readJul 22, 2022


July 20, 2022

Sleep’s important role in health and longevity has grown increasingly apparent. Sleep well nightly and you put off the Big Sleep.

I was a sickly kid. But now, at 74, my health is great, with no meds. I’ve also been fortunate to always follow a very regular sleep pattern. The two are evidently related.

We all know we’ve got built-in body clocks. But how they work, exactly, has been a tough scientific problem. I recently read a book by Steven Strogatz, Sync — The Emerging Science of Spontaneous Order, with a most interesting chapter on sleep.

Experiments have put volunteers in isolation rooms with no time clues. They’d sleep whenever. One researcher (Michel Siffre, in 1972) nearly went nuts partway in, begging to be let out. His collaborators outside disregarded this — dubious ethics, I think.

Anyhow, such experiments have shown our body clocks are not exactly 24 hours — typically a bit longer. But the subjects would not necessarily get into a sleep schedule resembling “normality.” Sometimes staying awake longer, and also sometimes sleeping longer. But here’s the interesting thing. The longer sleeps didn’t typically follow the longer wake intervals. Instead, a longer time awake is often followed by a shorter sleep. There seemed no rhyme or reason to this.

Our natural rhythms also include temperature fluctuations. Body temperature rises and falls during the day, seemingly separately from the body clock governing sleep. However, experiments have now actually revealed that the two are not unconnected. And our biological signal for hitting the sack is not feeling tired or sleepy — it’s when body temperature peaks. Going to sleep at that point in the cycle means sleeping long, with temperature now falling into a trough. When temperature starts rising again, that’s the wake-up alarm.

So even if you were tired after long wakefulness, if you go to bed when temperature will soon rise, that will wake you regardless. This is also the time when cortisol (a hormone) is being pumped out, raising alertness.

This pattern explains a lot of accidents, which tend to occur when people are at work in the wee hours, fighting their body thermometers, with brains not operating optimally. Thus TMI, Chernobyl, Bhopal, Exxon Valdez.

Ever notice how, if you stay awake for a long stretch, you become groggy? But if you push through it and keep awake, the grogginess dissipates and you get a “second wind.” What’s really happening is your body temperature rising back up. Likewise many of us feel drowsy in mid-afternoon. Guess what? Another temperature trough. Temperature is on a regular cycle.

“REM” refers to rapid eye movement, during sleep; it means we’re dreaming. Typically the longest and most intense dreams occur later during the night, before waking. But here again it’s been found that REM sleep does not follow the overall sleep time-picture. Instead, it too is governed by the temperature cycle — occurring just after the body is coldest. That’s why we often seem to wake up from a vivid dream.

But again the question is — how does this work? Do we have some sort of internal clock regulating it? Strogatz says that rather than having a clock, a person might be a clock. That is, such time-keeping is built into every component of the body. Body parts removed and kept alive in a dish still exhibit circadian rhythms.

Yet there seems to be a master clock regulating the whole system, apparently in the part of the brain called the hypothalamus. But how exactly that brain module performs that function remains something of a mystery.

Note that body temperature typically has peaked and is starting to fall just a couple of hours before a typical late evening bedtime. That’s what Strogatz calls a “forbidden zone” where it’s hard to fall asleep. Hence if you go to bed early — for instance, knowing you’ll have to get up early — it doesn’t work. This also accounts for a lot of insomnia — people trying to sleep at the “wrong” times given their body cycles.

But what about light and dark you say? You’re right. Daylight is indeed a powerful cue that keeps our body clocks constantly adjusting to the outside environment. This is especially important because as noted, our circadian rhythms are typically set on a schedule slightly longer than 24 hours. Why, is unclear. But without constant readjustment, we’d be haywire. Which in fact afflicts blind people, 80% of whom experience chronic sleep disorders. And the other 20% are apparently not so blind that their photo receptors can’t register any light at all — even if they cannot “see” it.