What is the progression of sleep stages in a usual night of sleep?
In the adult, NREM and REM sleep typically alternate in 90- to 120-minute cycles. Four to five cycles occur during a normal sleep period, depending on the length of sleep.
Each cycle is identifiable with sleep onset initiating in stage N1, progressing to stage N2, then to SWS (N3), back through N2, and into REM. Once the individual achieves sleep, there is not a significant return to stage N1.
Sleep cycling can also be thought of as alternating between REM and SWS, with N2 serving as a conduit between these two stages. In a typical night of adult sleep, stage N1 will comprise up to 5% of total sleep, stage N2 up to 50%, SWS up to 20%, and REM up to 25%.
SWS is predominately experienced in the first third of a night of normal sleep and REM in the last half of sleep.
Not achieving SWS or not achieving REM sleep has neuroendocrine significance. And for the individual, it is not just REM sleep or just achieving SWS that confers a rested feeling, but the full complement of > 7 hours of four to five cycles with normal architecture that is critical to restorative sleep.