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Why do humans practice monophasic sleep cycles?

When altering one's circadian rhythm to a Polyphasic sleep schedule, how much would that affect development?

Is Monophasic sleep (once per day) chosen because of biological evolution or is it due to cultural influence?

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The second question is quite different from the other two, and should probably be asked as a separate question. If it should be asked at all -- from the FAQ: "If you can imagine an entire book that answers your question, you’re asking too much." –  user24 Dec 27 '11 at 9:14
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You may be interested in this question over at Skeptics: skeptics.stackexchange.com/questions/999/… –  Larian LeQuella Dec 31 '11 at 6:07

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Reading "Polyphasic Sleep: Facts and Myths (Dr Piotr Wozniak)", it is pointed out that infant humans do undergo polyphasic sleep. As this is where most of our development is obviously done, I do not know where I can further proceed with the question about how it would affect development? Perhaps the issue is more how it would effect the day to day performance of a developed individual? If this is the case then it is suggested by Dr Wozniak that this is likely to be highly disruptive to the individual

Those well-defined effects of natural sleep affecting stimuli on sleep patterns lead to an instant conclusion: the claim that humans can adapt to any sleeping pattern is false. A sudden shift in the schedule, as in shift work, may lead to a catastrophic disruption of sleep control mechanisms. 25% of North American population may work in variants of shift schedule. Many shift workers never adapt to shifts in sleep patterns. At times, they work partly in conditions of harmful disconnect from their body clock, and return to restful sleep once their shift returns to their preferred timing. At worst, the constant shift of the working hours results in a loss of synchrony between various physiological variables and the worker never gets any quality sleep. This propels an individual on a straight path to a volley of health problems...

It appears that polyphasic sleep encounters the precisely same problems as seen in jet lag or shift-work. Human body clock is not adapted to sleeping in patterns other than monophasic or biphasic sleep.

It would therefore seem that polyphasic sleep is certainly detrimental to health, if not development.

However studies into cognitive performance resulting from differing sleep patterns run by Dr Claudio Stampi (Published ISBN 0-8176-3462-2), he concluded that polyphasic sleep was more efficient than monophasic sleep. Therefore it may be possible that polyphasic sleep patterns have no detrimental effect on development.

Individuals sleeping for 30 minutes every four hours, for a daily total of only 3 hours of sleep, performed better and were more alert, compared to when they had 3 hours of uninterrupted sleep

There are a couple of theories mentioned in the above book (beginning pg 5) which support the development of monophasic sleep as evolution rather than a social convention:

  1. Polyphasic sleep is regularly seen in smaller mammals that have very high metabolic rates, requiring them to spend most of their time foraging or hunting. Therefore a long sleep would be highly impractical for them as they would wake without the energy required to hunt their next meal. Humans do not have this need as they are larger and do not require such regular meals.
  2. Monophasic sleep would be beneficial for the early human hunter gatherer as our eyes are not well adapted to see at night. Therefore any time spent not using the daylight is wasted and any time without the light is not nearly as useful. This makes it more beneficial to sleep for an extended period when the sun is down.

I am sure that social factors would have an effect, however I would imagine the evolutionary pressures to be more significant.

I really would recommend the book (http://sleepwarrior.com/Claudio_Stampi_-_Why_We_Nap.pdf) if you have not yet encountered it as I found it very informative and packed with references to studies that you may find helpful.

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