Some mornings, waking up might feel like interrupting a vivid alternate universe. You open your eyes to reality, but the dream you were having still lingers clearly in your memory, complete with characters and plot points. Other days, waking up may be more akin to emerging from a black void with nothing to report. Even if you rarely recall details of your dreams, chances are you're still having them. Research indicates that nearly everyone dreams regularly–even those who claim they never do.

Non-rapid eye movement sleep, or NREM, is, collectively, sleep stages 1–3, previously known as stages 1–4. Rapid eye movement sleep (REM) is not included. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during these stages. Dreaming is rare during NREM sleep, and muscles are not paralyzed as in REM sleep.