There is such thing as sleep dependent memory consolidation. Without proper sleep, our brain is unable to fully recover from the events of the day, and unable to store memories as efficiently as it should. In REM sleep, important functions such as procedural memory are enhanced. REM sleep requires more time than just what a nap can provide. A nap may rejuvenate you for verbal memory and other minor functions, but does not fully refresh your mind like REM sleep does.