In psychology, we learn that there are 4 stages of sleep and REM
slp is the 4th stage. People will go through a cycle of sleep from
stage 1 to 4 back to 1 and each cycle is about 1.5hrs. As each slp
cycle pass, stage 1 and 2 sleep is slowly reduce till in the end no
more and stage 4 or REM sleep gradually increases. More REM sleep
actually makes you less tired if never remember wrong.