sleep stages
Summary
Sleep stages are phases of the ~90-minute sleep cycle, identified by EEG: non-REM stages N1, N2, N3 (deepest, slow-wave) and REM. Most testable: from N1 to N3 EEG frequency falls and amplitude rises, while REM looks like waking brain activity with muscle paralysis.
Detail
Meaning: Sleep is divided into three non-REM stages (N1, N2, N3) and REM sleep, distinguished by EEG patterns. An alert brain shows beta waves; a relaxed, eyes-closed brain shows alpha waves. N1 is light sleep with theta waves. N2 adds sleep spindles and K complexes. N3 is slow-wave sleep with high-amplitude, low-frequency delta waves. REM shows fast, low-amplitude activity resembling waking, plus rapid eye movements, vivid dreaming and loss of skeletal muscle tone. Older texts number the non-REM stages 1 to 4; stages 3 and 4 are now combined as N3.
On the MCAT: Passages show EEG traces or a graph of sleep across a night and ask you to name the stage from wave frequency and amplitude. A cycle lasts roughly 90 minutes and repeats several times a night. N3 is more prominent in early cycles, and REM periods lengthen in later ones. Other questions ask about REM rebound after REM deprivation.
Common confusion: REM is called paradoxical sleep because the brain looks awake while the body's voluntary muscles are essentially paralyzed. Do not extend the trend of slower, taller waves into REM: it breaks that pattern. Dreaming occurs in both REM and non-REM stages, though REM dreams tend to be more vivid.
Sources
- Psychology (Myers)
- Psychology (Schacter, Gilbert, Wegner)
- Discovering Psychology (Hockenbury)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.