v. Our 25-hour day?

21 November 2005 at 21:08 | Posted in Circadian rhythm | 5 Comments
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The myth of the built-in 25-hour day was started by scientists and they’ve had a devil of a time squelching it.  A few decades ago, scientists put themselves and other volunteers into caves and old mines to try to determine this.  They slept in total darkness, woke spontaneously, got up and were active and turned on the lights. 
Come evening they did not, of course, turn off the lights until they wanted to turn in.  Until recently it was thought that several thousand lux would be necessary to affect our circadian cycle.  It is now known that regular indoor lighting does have some effect.  So a few 60W bulbs in the evening would stretch the period to where it looked as though the average cycle was around 25 hours.
Such experiments are now done in very controlled conditions with regard to food intake, body position and activity as well as light.
The average cycle for human adults, young and old, is 24 hours and 11 minutes.  See, for example, Dijk and Lockley (2002) and a report by the National Institute on Aging (2000).  Letting your browser search for the number 25 will, in both of these articles, bring up the pertinent paragraphs. 
Why not exactly 24 hours?  Our bodies may be fine-tuned but they can be thrown off by stress or illness not to mention transmeridian travel.  So it wouldn’t have been very intelligent of Intelligent Design or whomever to not include a mechanism for adjusting.  Logical, then, to adjust (or control) just a tad every day. 
And why not a 23½ hour built-in day on average?  That could lead to awakening a bit too early and sleep deficit.  Though I’ve read that the average for some nocturnal critters is less than 24 hours. 
Next post:  vi. The timing of sleep
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