Computers, rats, and gila monsters all require precise internal clocks to function properly. Now, it appears, so do people. Researchers report in today's Science that our internal clock, or circadian rhythm, ticks on a precise 24 hour, 11 minute schedule. Contrary to conventional wisdom, the clock runs at exactly the same rate in older people as in younger people. By understanding how the internal clock normally works, doctors may be able to detect abnormal circadian rhythms responsible for sleeping disorders.
The brain's internal timekeeper, called the superchiasmatic nucleus (SCN), serves as a kind of master control for body temperature, hormone blood levels, and--most noticeably--desire to sleep. When deprived of sunlight and other cues, people's internal clocks seemed to vary from 13 to 65 hours, with an average of 25. But in 1996, Charles Czeisler, a neuroscientist at Harvard University, discovered that lightbulbs are bright enough to reset our internal clocks--and probably skewed the previous studies.
In a new study, Czeisler's team isolated 24 volunteers, some in their 20s and some in their 60s, for a month in a room with no external time cues that might reset their internal clocks. They lit the rooms with 10-watt lightbulbs that were turned on and off on a regular 20-hour or 28-hour cycle. (This was far enough off the natural rhythm to keep the internal clock from resetting itself.) After measuring hormone levels, body temperature, and recording sleep habits, the scientists found that circadian rhythms were much more consistent than in earlier studies: Only two volunteers had rhythms that deviated from the 24-hour, 11-minute mean by more than 10 minutes.
The finding reinforces the importance of a dependable internal clock, according to Robert Moore of the University of Pittsburgh, who discovered the SCN. "Clearly, nature would not go to the trouble of constructing such an elegant, functional, and durable timepiece if it was not critical to us throughout our lives," Moore says. He adds that Czeisler's methods might lead to better detection and treatment for people with "phase delay sleep disorder," a form of insomnia that typically begins in the teenage years.