Most of us don't notice our internal clocks until they're out of whack with the time of day, usually because of jetlag or a change in sleep patterns. Like a clock that needs to be set daily, the brain's circadian clock requires a cycle of sunlight and darkness to orchestrate the body's functions. (The term "circadian" comes from the Latin circa, "around," and diem or dies, meaning "day.") Almost every function of the body oscillates during the day according to this clock, affecting the timing of major events—like birth or even death. Chronic disruptions to our natural body rhythms can be a serious health hazard. Some studies have shown that night nurses with changing shifts are more likely to get breast cancer than those who work during the day, and shift work has also been linked to diabetes and obesity. But even those who are on more typical schedules can be affected by circadian rhythms. Here are more body clock believe-it-or-not facts from psychologist Roberto Refinetti, editor-in-chief of the Journal of Circadian Rhythms, and Steven Brown, of the University of Zurich.
1. Body temperature is lowest before waking up in the morning, and highest in the late afternoon. A temperature of 99 degrees Fahrenheit might indicate fever at 7 a.m., but by 5 p.m. would be normal.
2. Heart attacks are twice as likely in the early morning as at other times.
3. Births peak in the morning and early afternoon.
4. So do suicides.
5. Body clocks are why doctors often prescribe taking medicines at certain times of day. For example, both aspirin and antihistamines work best taken in the morning. But some types of chemotherapy for cancer may be more effective and less toxic if administered at night.
6. People are much more likely to have sex at night just before they go to sleep than at any other time of the day.
7. If we were to live in total darkness, we would fall into a cycle of days that last up to 25 hours or longer. "Clock genes" control body temperature, the secretion of hormones and much activity at the cellular level—all of which are controlled by a master clock that runs on the longer cycle unless it is reset by the sun.
(If you're a night owl who wants to adjust your body clock, read NEWSWEEK's tips on how to become a morning person.)