NREM Stage N1
This stage of non-REM sleep is the typical transition from wakefulness to sleep and generally lasts only a few minutes. Stage N1 is the lightest stage of sleep; patients awakened from it usually don’t perceive that they were actually asleep
During this stage:
Eye movements are typically slow and rolling.
heartbeat and breathing slow down
muscles begin to relax
you produce low amplitude mixed frequencies waves in the theta range (4 to 7 Hz)
NREM Stage N2
This next stage of non-REM sleep comprises the largest percentage of total sleep time and is considered a lighter stage of sleep from which you can be awakened easily. This is the stage before you enter deep sleep.
During this stage:
heartbeat and breathing slow down further
no eye movements
body temperature drops
Sleep spindles and K-complexes are two distinct brain wave features that appear for the first time
NREM Stage N3
This final stage of non-REM sleep is the deepest sleep stage. Stage N3 sleep is known as slow-wave, or delta, sleep. Your body performs a variety of important health-promoting tasks in this final non-REM stage.
During this stage:
arousal from sleep is difficult
heartbeat and breathing are at their slowest rate
no eye movements
body is fully relaxed
delta brain waves are present
tissue repair and growth, and cell regeneration occurs
immune system strengthens
REM Stage R
There are two phases of REM sleep: phasic and tonic. Phasic REM sleep contains bursts of rapid eye movements, while tonic REM sleep does not.
Stage R occurs about 90 minutes after you fall asleep, and is the primary “dreaming” stage of sleep. Stage R sleep lasts roughly 10 minutes the first time, increasing with each REM cycle. The final cycle of stage R may last roughly between 30 to 60 minutes.
During this stage:
eye movements become rapid during phasic REM
breathing and heart rate increases and become more variable
muscles become paralyzed, but twitches may occur
brain activity is markedly increased
When you fall asleep at night, you cycle through all of these stages of sleep multiple times — roughly every 90 minutes or so.