What about polyphasic sleep? If the study only look at normal sleep, then obviously the ones who sleep less than 7 hours will be less likely to remember things

What about polyphasic sleep? If the study only look at normal sleep, then obviously the ones who sleep less than 7 hours will be less likely to remember things than the ones who had a full night sleep. REM sleep is important, but REM sleep does not cover all 7 hours (more like about 2 hours). So if you get to REM sleep faster, you won't need to sleep as much.