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I saw this cute online sleep calculater. They claim (put in any time, and you'll see it):

Sleep cycles typically last 90 minutes.

Waking up in the middle of a sleep cycle leaves you feeling tired and groggy, but waking up in between cycles lets you wake up feeling refreshed and alert!

They appear to claim that if, for example, you need to choose between sleeping 7 and a half hours (which is a multiple of 90 minutes), or sleeping 8 hours, then 7 and a half would be better for feeling awake in the morning. Is this correct? In particular, a doctor I spoke to said that more sleep is more important than not interrupting sleep cycles.

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    @Articuno I think it was for feeling alert during the day, not so much as upon waking. And it was a female doctor, btw. – ike Jan 23 '14 at 15:49
  • I think the claim is well-defined. – ike Jan 24 '14 at 20:31
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    OFF TOPIC SLIGHTLY. This is a very interesting article on sleep deprivation if you are interested => creepypasta.wikia.com/wiki/The_Russian_Sleep_Experiment – user13859 Jan 26 '14 at 13:09
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    Whenever I tried it I was refreshed, and when I didn't follow it I was groggy. The best is 6 hours or 7.5 hours (but never 6.5 or 7). If you need to sleep shorter, go for 4.5 hours or 3 hours - even if you show signs of lacking sleep, you'll still feel better and not lacking energy. If anecdotal evidence is any good, I vouch that this is true. – ADTC Jan 29 '14 at 9:08
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    Remember that individual variances of just a few minutes (John has cycles of ~87 minutes, Jane has cycles of ~93 minutes), multiplied by 5-6 cycles per night, will lead to significantly different "best waking" times. So don't rely on online calculators: measure your own behavior. P.S. Related skeptics.stackexchange.com/questions/4864/… – Larry OBrien Jan 29 '14 at 18:05
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See this Nature blogpost, where one conclusion is this:

Don't even worry about waking up during the "right" stage of sleep. Research by Jewett and colleagues at Harvard suggests that the stage of sleep we've been woken from, despite what we perceive as sleep inertia, does not actually have an impact on cognitive performance.

Note though that the correct answer is probably "We don't know." Sleep is one of the more poorly-researched areas in medical science. Perhaps this is why we have these conflicting claims.

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Sleep cycles typically last 90 minutes.

This is not entirely true. The first stage sleep cycles do but they become longer through the night.

Wikipedia cites Michael Billiard. Sleep: Physiology, Investigations, and Medicine (Google eBook). Springer. p. 5. as saying:

The average length of the first sleep cycle is approximately 90 minutes and 100 to 120 minutes from the second to the fourth cycle, which is usually the last one.

This conflicts with the proposed maths.

I have not heard of any conclusive scientific study on the optimal cycle to wake up on, although in my own experience I much prefer to wake up during a sleep cycle. This guarantees that you will be able to recall the dream you were having before you woke up.

It is very much recommended however that you should not set an alarm clock to wake you up. As this interferes with the body's natural process of waking up (hard to avoid however in our world though) . REF

  • Whoops. How did we let past a reference to Wikipedia as the main source? – Oddthinking Apr 1 '14 at 8:55
  • About the alarm clock part - it's only my and my friends experience, but it takes only a week or two of going to sleep and waking up at the same hour each day for your body to wake itself up before the alarm clock. – Maurycy Aug 3 '14 at 20:37

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