Do you live by the clock, checking your watch so you're not late for work, school or appointments? While you struggle to stay on time for your daily activities, your body has its own internal clock. This biological clock helps you feel alert at work, hungry at mealtime and drowsy at night. When your internal clock is out of whack, your health can suffer.
Scientists have long known that the human body has daily rhythms, called circadian rhythms (circadian is Latin for "around a day"). These natural rhythms are coordinated by a tiny region in your brain. This "master clock" generally operates on a 24-hour cycle, and it adjusts to several cues in your surroundings. The most important cue is light and darkness.
During the day, when it's light outside, the brain's master clock sends out signals to other brain regions to make hormones that will help keep you awake, boost your heart rate and give you energy. In the evening, when less light enters your eyes, the master clock triggers production of a hormone called melatonin. Melatonin makes you feel drowsy and helps you stay asleep.
The brain's clock affects various body functions, including body temperature, hormone levels, urine production and blood pressure.
"Many processes are patterned around a 24-hour cycle: sleeping, eating, waking, drinking and even health-related events," says Dr. Martha Gillette, a scientist at the University of Illinois at Urbana-Champaign. For instance, she notes, heart attacks are more likely to occur early in the morning, when the level of a hormone called cortisol starts its daily rise. Time of day has also been shown to influence the effectiveness and side-effects of certain drugs.