i can say from my personal experience. if i read hard staff like parasitology once and get nice 8 hr sleep, i can memorize and remember everything second day. and once i had mid term test and wanted to study hard and decided to study whole night and you know what? i could not remember single thing during exam :( i and was mixing everything i have read. my opinion is sleep organizes every information you got previous day and without it you waste your time and energy.