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Let's say, hypothetically, that you fell asleep at 10PM, and you were woken up by a noise at 1AM. If you went back to sleep immediately and woke up at 6AM, you will feel tired even though you had slept for 8 hours. However, if you sleep for 8 hours straight , your body will feel rejuvenated. Is it because of REM sleep being cut off and restarting again?

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Yes, but I'm guessing. OTOH, you can be well rested after 1 hour, if you know beforehand (=expect) that it will be short. So, it's more a question of psychology, not biology IMHO. –  rwst Oct 6 '12 at 5:58
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Sleep is not homogenous - there are different phases of sleep - REM, NREM-1 to NREM-4. And these different stages may play different roles in 'rejuvenating' the body (as you put it).

When you first fall asleep, you go into NREM-1 phase, which is the lightest sleep, and progresses into NREM-4, and for the first half of your sleep cycle, this cycles so you go from NREM-1 > NREM-2 > NREM-3 > NREM-4 > NREM-3 > NREM-2 > ....

On the first half of your sleep cycle, the NREM3 and NREM4 phases dominates, only on latter half of your sleep cycle are REM phase observed and progressively dominate.

Depending on how much you are waken up by, once you escape your sleep cycle, you may re-enter back at a different stage, which may mean you are no longer able to have much REM sleep. Subsequently, you will not have the rejuvenation that is associated with REM sleep and you feel tired.

Interesting note: There are people who have tried a method called polyphasic sleep, where they nap/sleep periodically and regularly, instead of sleeping once a day for 8 hours, say. People who promote this says that people do not feel tired because as soon as they nap, they go in REM sleep, and so the amount of REM sleep you get per day will still be the same. I'll let you make your own mind up about this one.

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