But these external clocks don’t exactly match the biological tickings inside ourselves. Many measurements led to the conclusion that the internal clock period is actually closer to 25 hours; that is, the biological clock was thought to drift toward a 25-hour day unless it is set back an hour each day by exposure to morning light and to external clocks. This situation is blamed for a long list of sleep problems.

Now, the most accurate measurements to date, made by researchers at Harvard University, reveal that our natural daily rhythm is much closer to that of other living things than previously believed. The better match opens the way for experts to more effectively treat sleep problems involving night work, jet lag, Earth-orbiting astronauts, or just not being able to go to sleep and wake up on time.

By recording the daily rhythms of hormones and body temperatures in 24 healthy young and old men and women over a one-month period, the researchers conclude that our internal clocks run on a daily cycle of 24 hours, 11 minutes.