You and every other living thing are slaves to the clock. Not to the numbers on your watch dial or smartphone but to a more demanding master: the body clock. Your sleep/wake cycle, body temperature, mental agility, moods, athletic performance, appetite, sexual desire, and more all wax and wane on a regular cycle known as a circadian rhythm. (Circa from the Latin for “around” and dian from the Latin for “day,” so “around a day.”)
The sleep/wake cycle is the most obvious daily rhythm, and for most of history people logically assumed that it was dictated by daylight. A gloomy experiment in 1938 proved this wrong. University of Chicago sleep scientist Nathaniel Kleitman and an assistant, Bruce Richardson, camped out for 32 days in the total darkness of Kentucky’s Mammoth Cave to see how their bodies would respond in the absence of external cues.
What they found: Even without daylight their bodies adhered to regular cycles of sleep and body temperature ups and downs. However, the cycles were not precisely 24 hours long. Over time, they gradually lengthened to somewhere between 24 and 28 hours.
The clock of sleep
Today we know that every human body is governed by a biological clock in the brain, a tiny region called the suprachiasmatic nucleus.
However, outside cues such as light, temperature, and meals also influence circadian rhythms. Known as zeitgebers (from the German for “time giver”), these cues help to keep us on a roughly 24-hour schedule that corresponds to night and day.
(Discover the amazing journey our mind goes on when we sleep.)
Bolstering the sleep/wake rhythm are several chemicals that the body releases in a daily cycle. Among them are adenosine and melatonin. Adenosine is a drowsiness compound, building up in the brain during the day and increasing your need to sleep. When you sleep, adenosine levels fall, only to rise again the next day. Melatonin, a hormone released in the evening by the pineal gland, cues the brain that sleep is (or should be) imminent.
Although these mechanisms operate in every human body, each person’s clock is set a little differently. “Larks” are best and brightest in the morning, rising early and sleeping early as well. “Owls,” the late risers, are slow to get going but still lively late at night. Circadian rhythms also affect body temperature and hunger.
Understanding your own body’s rhythms can have a positive effect on your daily performance. The majority of people are most mentally alert in late morning and early evening, and sleepiest in early afternoon, when they’re ready for a post-lunch nap. Athletic performance peaks in the afternoon and early evening. Bright lights at night, including the glow from TVs and handheld screens, are zeitgebers that trick the body into wakefulness.