What Is Deep Sleep?
Deep sleep, also called slow-wave sleep, occurs in the third stage of non-rapid eye movement (NREM) sleep. During deep sleep, electrical activity in the brain appears in long, slow waves called delta waves . These waves have a frequency of 0.5 to 2 Hertz and they must make up at least 6 seconds of a 30-second window for that window to count as deep sleep.
Typically you descend into deep sleep within an hour of falling asleep, and experience progressively shorter periods of deep sleep as the night wears on. During this stage, automatic body functions like breathing and heart rate are also very slow and your muscles are relaxed. It can be difficult for someone to wake you up, and waking up out of deep sleep may make you feel mentally foggy for up to an hour.
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