On Jun 7, 2006, at 2:01 AM, Clive D.W. Feather wrote:
Actually, the evidence from experiments is that the "natural" sleep- wake cycle is about 27 hours long, but force-locked to the day-night cycle (it's easier to synchronise a longer free-running timer to a shorter external signal than vice-versa).
References for this? Your explanation makes a lot of sense and I'm prepared to be convinced, but have been skeptical of experimental design as applied to questions of human behavior since participating in studies as a requirement of undergraduate psychology coursework. And if this cycle is inferred from the behavior of undergraduates, I'm even more skeptical :-)
So humans will cope until the solar day is about 27 (present) hours long, after which we'll probably start to move to a system of two sleep-wake cycles per day.
Well, "cope" isn't the right word if this cycle is as you describe. Also hard to imagine how one gracefully transitions from one to two sleep cycles a day. It would simply appear that the underlying mechanism evolved to rely on a slightly longer free-running timer synchronized to length-of-day. As the day lengthens, Darwin would predict that our intrinsic cycle would also lengthen. This is similar to arms races leading to other periodic natural behavior such as prime number 13 and 17 year locusts. (Non-primes would allow locust predators to emerge more frequently while locking into the phase, thus gaining an advantage.) Also, whether or not one believes that humans have somehow escaped the grip of evolution, it is hard to imagine our continued sojourn on Mother Earth half a billion years hence :-) Obvious lines of research for further sleep period investigations would be to examine similar cycles in other animals. One imagines this is some function of the nervous system, so one might also contrast strategies pursued by plants and animals without. Also - how is this intrinsic cycle inferred? Could signatures of this intrinsic cycle be preserved in the fossil or DNA record? All sorts of other cycles are. Could such signatures be correlated with length- of-day at various epochs? Just another mechanism tying our species to time-of-day. Rob NOAO
