science of us

How Your Body Keeps Time

Photo: Chad Baker/Getty Images

Whether you know it or not, you have an internal body clock that follows a daily 24-cycle known as a circadian rhythm. Our body clock, also known as a circadian clock, responds primarily to environmental cues, such as light and darkness, as well as genes that influence how quickly or slowly our body clock runs and, as a result, how closely it aligns with the 24-hour day.

Nearly every tissue and organ in the body is governed by its own circadian clock, which is responsible not only for controlling our sleep patterns, but for synchronizing every biological process — our temperature, our blood pressure, our hormones, to name just a few — to the time of day. A master clock, controlled by the brain, keeps all the clocks in sync.

When we are in good health and keeping regular sleeping and eating schedules, our bodies really do run like clockwork. Our body temperature rises just before dawn, helping us feel alert when we wake up; it then dips at night, promoting sleep. But when our body clock is disrupted, whether by occasional or continuous interruptions of sleeping patterns, and our natural circadian rhythm is either slowed or accelerated, all sorts of stuff can go wrong.

Three doctors — Jeffrey C. Hall, Michael Rosbash, and Michael W. Young — were awarded the Nobel Prize in Physiology or Medicine this week for their pioneering discoveries on the circadian clock. Their contributions have laid the foundation for an entire field of science dedicated to the study of our body clocks and circadian rhythms. Decades later, scientists are still unraveling how our inner timekeeping affects health and disease. Here’s what we know so far.

The time of day you work matters. Several studies, including the Nurses’ Health Study, have found an increased risk of cancer among night-shift workers, including breast cancer and colorectal cancer. Researchers concluded that that increased exposure to light at night, rather than following a normal sleeping pattern, resulted in a reduction of the hormone melatonin, which may lead to cancer growth.

So does the time of day athletes compete. There is, in fact, a best time of day to go for the gold — so much so, according to Aeon, that many records are broken in later in the day when the body temperature is higher and muscle contractions and flexibility are at their peak. Research has shown, for example, that soccer players’ technical skills, including both juggling and dribbling, are better in the afternoon than in the morning, as are swimmers’ speeds.

“Resetting” the body clock may help people with depression. A recently published analysis of more than 2,000 sleep studies has shown that sleep deprivation may help alleviate depression. Though study authors aren’t sure why, a prevailing theory is that some people with mood disorders may have a malfunctioning body clock, which a lack of sleep could help reset.

Heart attacks are more likely in the morning. Mounting evidence suggests that circadian clocks are closely related to heart health. The diurnal changes in blood pressure and heart rate are well known circadian rhythms. Multiple studies have also shown that stroke, sudden cardiac arrest, and heart attacks more commonly occur in the morning, with one study showing that heart attacks are two to three times as common during the first waking hours than at night.

It is (maybe) true what they say about eating late at night and weight gain. A growing body of research now links nutrition and obesity with the circadian clock. One recent study found that changes or disruptions to the circadian clock not only change eating times but also result in weight gain. Study authors caution that people who work night shifts have an increased risk for obesity, diabetes, and heart disease as a result of unusual eating times and disruption of circadian rhythm.

Catching the flu in the morning may result in a worse infection. Most of us have a time of year when we’re more likely to get sick. As it turns out, there may also be a time of day. A study published last year in the Proceedings of the National Academy of Sciences found that viruses reproduce ten times faster in animals infected during sunrise than those infected later in the day, leading to a worse infection. This may be because the body is less prepared to defend against germs at certain times of the day than others. More research is needed to determine infection at the wrong time influences how disease susceptibility and its potential severity.

There’s a reason your skin is itchier at night. The skin, like other organs in the body, follows its own internal clock. The skin’s temperature, pH balance, and oil production vary by the hour. Even the appearance of wrinkles varies by time of day. At night, its protective barrier, which guards against bacteria, viruses, and other irritants, also becomes easier to penetrate, which may explain why if you’re feeling itchy, it’s most likely after sundown.

A disrupted sleep schedule may affect fertility. A growing number of studies suggest disruptions to circadian rhythm caused by jet lag or late-shift work may have negative effects on a woman’s fertility. One published last year in PLOS showed that mice, which have similar biological-clock genes as humans, with altered sleep patterns had more difficulty becoming pregnant and carrying pregnancies to term. More research is needed to determine whether findings from these animal studies have implications for women’s reproductive health.

Weird sleeping patterns can lead to migraines, too. Any disturbance from the normal routine of sleep, be it staying up too late or getting up earlier than usual, can trigger a migraine. And, most migraine sufferers are relieved only after getting some sleep. That’s not a coincidence. Researchers at UCLA recently linked the debilitating condition to a mutation in a gene that helps regulate circadian rhythms.

How Your Body Keeps Time