Many studies led to the conclusion that the internal body clock period (circadian rhythm) is actually closer to 25 hours; that is, the 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 (Cromie, 1999).

However, as Cromie (1999) points out, the most accurate measurements found that our internal clocks run on a daily cycle of 24 hours, 11 minutes ± 16 minutes (Czeisler, et al. 1999).

“It was both surprising and reassuring that the human clock runs with the same precision as that of other living things,” Czeisler comments. “But we did not expect to find that both young and old people have the same daily period.” (Cromie, 1999).

Czeisler, who is also director of circadian and sleep disorders medicine at Brigham and Women’s Hospital in Boston, said that the reason these measurements were more accurate was because:

Other investigators used elaborate procedures to shield subjects from time cues and the outside world. They even coiled copper wire around the rooms in which people were isolated to counteract natural electromagnetic fields. However, these experimenters allowed their subjects to switch on lights when they were awake and turn them off when they wanted to sleep. They didn’t think this would have any effect, but switching on electric lights resets the biological clock. It’s the same as resetting your watch (Cromie, 1999).

Have similar studies been conducted since 1999 with a larger test group and were the results the same? What are the latest ideas to explain the earlier wake-up times of older people?

This is especially when biological clock lore states that we drift to a later wake-up hour on weekends because we fail to reset the 25-hour cycle each morning as we go to work (Cromie, 1999) and Czeisler also pointed out that

We’re not drifting, we’re pushing ourselves to a later time with our exposure to electric lights from sunset to bedtime. That resets our biological clocks.”


Cromie, W. J. (1999). Human Biological Clock Set Back an Hour. Harvard Gazette. Retrievable from: https://news.harvard.edu/gazette/story/1999/07/human-biological-clock-set-back-an-hour

Czeisler, C. A., Duffy, J. F., Shanahan, T. L., Brown, E. N., Mitchell, J. F., Rimmer, D. W., ... & Dijk, D. J. (1999). Stability, precision, and near-24-hour period of the human circadian pacemaker. Science, 284(5423), 2177-2181. doi: 10.1126/science.284.5423.2177

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