How does your body know what time it is, even in the dark?

Your body knows the time because it carries its own clock, which keeps running even without daylight. Biologists call its daily cycle a circadian rhythm, from the Latin for 'about a day'. In mammals the master clock is a tiny cluster of cells in the brain's hypothalamus, the suprachiasmatic nucleus. It reads how light or dark it is through the eyes and passes the news to the pineal gland, which releases the hormone melatonin at night. Melatonin is practically absent by day and starts to rise in dim light around 9 p.m.

The clock runs on its own, but not perfectly: left alone it drifts. So it is reset every day by outside cues, mainly light, which scientists call zeitgebers, German for 'time givers'. When you fly across time zones, your clock is still on home time, and jet lag lasts until the new daylight has pulled it into line.

The first hint of such a clock came from a plant. In 1729 the French scientist Jean-Jacques d'Ortous de Mairan noticed that a mimosa kept opening and closing its leaves on a 24-hour schedule even when kept in constant darkness. And a popular myth says the human clock runs on a 25-hour day. That figure came from early studies whose volunteers could switch on electric lights; a stricter 1999 study found about 24 hours and 11 minutes.