As stated earlier, it’s kind of a mystery. However, science has pointed out the fact that if people don’t get all four stages of sleep several times a night - life stinks. (To sum it up.) Both the REM and NREM sleep stages play important roles in our mental and physical health. Each stage plays in own special role in restoring our bodies and brains, so when you skimp on one or more sleep cycles, you're not giving your body the rest it needs.
REM sleep stimunlates the areas of your brain that help you learn and retain memories. Also, according to the National Institute of Neurological Disorders and Stroke, rats deprived of REM sleep lives much shorter lives. And rats who weren’t allows to enter the the sleep cycles at all? Well, lets just say there wasn’t a whole lot of living going on at all.
In the end, REM sleep, where you dream and commit things to memory, is where your brain completes critical neural connections that help you feel more rested and, well, more human during waking hours. (Plus, you’ll remember your daughter’s best friend’s boyfriend’s uncle’s name - BONUS!)