Since your brain desires most of its REM sleep in the last part of the night, which is to say the late-morning hours, you may lose 60 to 90 percent of all your REM sleep, even though you are losing 25 percent of your total sleep time. It works both ways. If you wake up at ten a.m., but don’t go to bed until four a.m., then you will lose a significant amount of your normal deep NREM sleep.
