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. 
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Next post:  vi. The timing of sleep
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  1. So that’s why you always hear that the natural cycle is 25 hours: that’s how long people tend to stay up if they use artificial lighting, which delays melatonin onset. Of course. I couldn’t work out how we could have evolved in such a crazy way. Any idea why blind people are prone to N24, then?

  2. The crazy way we have evolved is still a longer-than-24-hour day. I saw an explanation for that which may be correct.
    1. It shouldn’t be exactly 24 hours as it has to be adjustable if thrown off, f.ex. by fever.
    2. It shouldn’t be less than 24 hours as we might be awakened too easily too early, and be sleep deprived. (The argument seems minor, compared to what people do to themselves. But it holds also for all diurnal animals. Nocturnal animals generally have a shorter-than-24-hour day built in.)

    Your N24 question may be answered by the new author on this blog. I was just going to ask an N24 question in a comment to his first post here.

  3. I don’t follow the logic of: “it shouldn’t be exactly 24 hours as it has to be adjustable”. Can you clarify this? What does the natural cycle being exactly or not exactly 24 hours have to do with being adjustable or not? To me, these seem independent properties (exactly or not exactly 24 hours, and adjustable or not adjustable).

  4. I see what you are saying. It may well be exactly 24 hours for some, but we wouldn’t want to have to count on its accuracy our whole lives without adjustment possibilities. (And certainly not if we wish to travel to different time zones.) In general, animals which sleep at night have natural cycles a bit longer than 24 hr (24.2 for humans). I’ve read that one reason might be to keep us sleeping heavily enough to not be too easily awakened before we’ve had enough sleep. Thanks for commenting. (Ooops, I didn’t read the blog post nor the comment before yours, before answering. As you can see.)

  5. Insearchofmornings, I see you didn’t get an answer to your N24 question. Perhaps you’re still subscribed to this post. About half of totally blind people have N24, because they live their longer-than-24-hr days, not adjusting them by daylight. Some blind people don’t consciously perceive light, but functioning light sensitive cells in their retinas entrain them to 24 hours. Some even have pupillary light reflex, even though they cannot see.


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