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An old friend once told me about sleep cycles and how each cycle lasts about 90 minutes. Within those 90 minutes, you go from light sleep, all the way to REM and back to light sleep. He said that it was easy to wake up if you time the cycles correctly (i.e., if you sleep exactly 1.5 hours, 3 hours, 4.5 hours, 6 hours, etc...), but if you wake up in the middle of a cycle, especially if you were in REM sleep, you will feel very tired since you did not complete the cycle.
The guy was kind of strange, but what he was saying made perfect sense lol.
It's kind of funny because sometimes I'll wake up at 3am (I usually go to bed at 11:30, but I assume that by 12, I'm gone) and I feel perfectly rested, but I go back to sleep since 3am is too early to wake up.
... maybe he was right ...
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