You get sleepy at about the same time every night and hungry at about the same times every day. Some people even wake up just before the alarm goes off. It's as if there's a clock ticking somewhere inside you. As it turns out, there is.
Your body runs on what scientists call a circadian rhythm, a natural cycle that repeats roughly every 24 hours. The master clock sits in a pair of tiny cell clusters in the brain called the suprachiasmatic nucleus, or SCN, in a region called the hypothalamus. It takes cues from daylight through your eyes and uses them to keep the rest of your body on schedule.
About a Day, Not Exactly a Day
The word circadian comes from Latin roots meaning "about" and "day." That "about" matters. When people are cut off from every time cue, their internal clock keeps running, but not at precisely 24 hours.
Early experiments suggested people naturally preferred a day close to 25 hours. But those studies had a flaw: participants could switch on indoor lights whenever they liked, and evening light pushed their clocks later. A more careful 1999 Harvard study estimated the natural human rhythm at closer to 24 hours and 11 minutes. A 2010 study found that women's cycles ran slightly shorter than men's on average.
Light Sets the Clock
Since the inner clock drifts a little, it needs resetting every day. Light is the main signal. Your eyes contain the familiar rods and cones for seeing, but also special light-sensitive cells that send signals straight to the master clock in the brain.
The brain then passes the message to the pineal gland, a tiny structure shaped like a pine cone. It releases the hormone melatonin, which peaks at night and ebbs during the day. In dim light, melatonin typically starts rising around 9 p.m.
Light isn't the only signal. Mealtimes, physical activity, temperature, and stress can all nudge the body's clocks. Scientists call these time-setting cues zeitgebers, a German word meaning "time givers."
A Clock in Nearly Every Cell
The brain's clock is the boss, but it isn't alone. In 2013, neuroscientist Joseph Takahashi and colleagues wrote that "almost every cell in the body contains a circadian clock." These smaller clocks have been found in the liver, lungs, pancreas, skin, and other organs, and mealtimes play a key role in keeping some of them on track.
When all these clocks fall out of step, you feel it. Fly across several time zones and you may get jet lag, with tiredness, trouble sleeping, poor appetite, and headaches until your body catches up with local time. If you have ongoing trouble sleeping, talk to your doctor.
From Mimosa Leaves to a Nobel Prize
People have been curious about inner clocks for centuries. In 1729, French scientist Jean-Jacques d'Ortous de Mairan noticed that the leaves of a mimosa plant kept opening and closing on a daily schedule even when the plant was kept in constant darkness. It was an early clue that living things carry their own timekeepers.
Much later, in 1971, researchers Ron Konopka and Seymour Benzer found the first clock gene in fruit flies and named it "period." Work building on that discovery earned Jeffrey Hall, Michael Rosbash, and Michael Young the 2017 Nobel Prize in Physiology or Medicine.
Daily rhythms show up in animals, plants, fungi, and even some bacteria. So the next time you yawn at the same hour every night, give your inner clock a little credit. It's been keeping time all day long without you ever having to wind it.
