REM or rapid eye movement sleep is a stage of sleep. REM sleep usually happens late in the sleep cycle. During REM sleep, you have lots of dreams and your eyes actually move rapidly under your eyelids! That is why it’s called rapid eye movement sleep. REM sleep only happens a few times in the sleep cycle. The rest is NREM sleep or non-rapid eye movement sleep. Your brain is very active during REM sleep. That is the time when you dream and also sort through information that you learned/collected in the past day. REM sleep is like your body’s recovery mode, where it is cleaning up and fighting off bad bacteria or viruses! That is why REM sleep is so important! That is how the brain is as active as it is awake during REM sleep!