The Circadian Rhythm

The Inevitable Ups and Downs of Daily Life

Even if you’ve never heard the term “circadian rhythm,” you probably already know the concept like the back of your hand. We all feel the push-and-pull of this natural 24-hour cycle exerting a kind of tidal influence on our behavior throughout the day. An influence that controls our energy level, mood, eating patterns, and of course sleeping patterns. And we sense that this has to do with something more than just habit or routine. That, in fact, these patterns are controlled internally by a sort of “biological clock” working deep inside our bodies.

It turns out that our daily schedules are imposed on us from within as much as from without.

You get a reminder of how deeply embedded this cycle is when you try to change it abruptly. Just think of how you feel when you eat a meal at an odd time, get up extra early in the morning or stay up past your normal bedtime (or for that matter, try to go to bed earlier than usual).

The definitive example of this is, of course, jet lag. Hop on a plane and fly from, say, Los Angeles to London, and you might find yourself in quite a struggle to readjust. It may be mid-morning there, but your body isn’t having any of it. Big Ben says 10:00 A.M., but your internal clock says 2:00 A.M., and all you want to do is sleep. When night comes and you finally can sleep, you’re wide awake because your body thinks it’s daytime. This is your innate circadian rhythm at work.

In our quest for better sleep, a good understanding of the circadian rhythm is a biggie. You’ll want to know how it works, learning to recognize in yourself its clockwork patterns so you can synchronize your daily life with its natural ebb and flow. When you’re in tune with the rhythms of your own body, they’ll carry you along like a rising tide. Trying to go against these rhythms, though, usually spells trouble.

So let’s go in for a closer look…

What is it?

A circadian rhythm is a regular, daily cycle, with an approximately 24-hour period, which controls or influences virtually every biological function of a living organism. It is present in all living things — everything from plants and animals to fungi and bacteria.

It is endogenous, meaning that it originates from within the organism and is not dependent on environmental cues; it will run freely and pretty accurately even if isolated from all outside influences. But it does utilize environmental cues in part — most notably light — in order to keep itself in sync with its surroundings. Think of a clock that runs independently, but is reset every day to maintain perfect accuracy.

Modern scientific understanding of the circadian rhythm starts in 1729 with an experiment performed by the French astronomer and geophysicist, Jean de Mairan. De Mairan had observed the heliotropic cycles of mimosa plants as they opened and closed their leaves at the same time each day. He wanted to know if the plants were reacting directly to the sunlight or if they were following some internal cycle. So he took the plants and locked them away in the dark, completely isolated from the influence of the sun.

After a few days, he had his answer: the plants continued their cycle of opening and closing even in constant darkness. Unfortunately the results of De Mairan’s experiment didn’t take the world by storm and it would be more than two hundred years before the circadian rhythm became established as scientific fact.

A Sense of Time

For most of the twentieth century, our daily patterns of sleep and activity were understood mostly in terms of behavioral conditioning,. Our bodies and minds were just passively responding to external stimuli. This was the same idea behind Pavlov’s dogs. You know the story: The Russian physiologist Ivan Pavlov who demonstrated that by ringing a bell just before feeding his dogs, he had conditioned them over time to salivate anytime they heard a bell ring.

A similar mechanism was assumed to be at work in our sleep-wake patterns. Our desire to sleep at night was little more than a conditioned response to certain environmental cues such as the coming of dusk, the nighttime drop in temperature, eating dinner, certain social interactions perhaps — along with the waning away of visual and auditory stimulation as our surroundings grew dark and quiet. And with the return of light early the next morning, we were similarly brought back to wakefulness and activity. There’s no doubt that such cues do play a role here, but we now know that there is something more fundamental behind all this.

In the 1960’s, the Max Planck Institute in Germany carried out an ambitious experiment to test the circadian rhythm hypothesis in humans. They recruited volunteers to live for extended periods in total isolation from the outside world and any environmental time cues. The institute set up an underground facility and thoroughly insulated it from all outside light and noise so that the subjects could never be sure whether it was day or night. They went so far as to shield it from daily changes in ambient electromagnetic waves by wrapping its outer walls with copper wire. Male researchers working in the facility were even made to shave at irregular hours to eliminate the visual clue of five-o’-clock shadows.

The results were conclusive: the test subjects who lived in the isolation chamber continued to exhibit a definite circadian rhythm in their sleep-wake patterns even in the total absence of outside cues.

25-Hours a Day

In these early experiments, scientists noticed a curious thing: the human circadian rhythm, under free-running conditions (i.e. independent of any time cues), seemed to have a period of not 24 hours, but 25 hours!

For the next thirty years, the 25 hour circadian rhythm stood as conventional wisdom in science and took hold in the public imagination as well, becoming one of those popular factoids people drop on each other at cocktail parties. But as with so many other bits of conventional wisdom, this was destined to be disproved. The best current understanding has the free-running cycle with a period of more like 24 hours and 10 minutes — still somewhat slow, but much closer to an earthly 24 hour day.

What caused the test subjects in those early experiments to have 25 hour circadian rhythms? The presence of artificial light. When those first tests were conducted in the ’60’s, the thinking was that artificial light was of such a low intensity that its effect on the circadian rhythm would be negligible — so researchers didn’t control for it.

But further studies in the ’80’s and ’90’s, carried out by Dr. Charles Czeisler at Stanford University, revealed that artificial light can and does influence the circadian rhythm. In later experiments that were specifically designed to account for the effect of indoor lighting, the 24 hour 10 minute period emerged as the answer.