REM sleep is a critical stage of the sleep cycle. It’s when you get deep, restorative rest, but it’s not as hard to rouse from as deeper sleep. Each sleep cycle lasts about 90 to 110 minutes ...
Napping can make it difficult to go back to sleep at night. Long naps might also cause grogginess, resulting from waking up from deep sleep. If you must nap, aim for less than 30 minutes.