Researchers have found that non-REM sleep actually helps strengthen memories, while REM sleep cleans them up and prevents them from overlapping. Humans have evolved a sleep cycle that segues from non-REM (NREM) to REM sleep and back again throughout the night. (While it is possible to dream during NREM sleep, REM sleep is when we experience some of our most hyperrealistic dreams.) Once this stage is complete, the brain will then transition back to NREM sleep, and the cycle restarts. Related Story Scientists Found Life After Death—as a Mysterious Third StateUntil now, scientists could not figure out what kind of biological function this sleep cycle had.