IMPORTANT NOTE: This is not a medical site. You should always discuss medical matters with your doctor.
The Stages of Sleep
People often consider sleep a simple matter of "switching off". We slowly
drift off, then once we are "asleep" we remain that way until we wake up. In reality sleep is a lot more complex than that.
The Sleep CycleA normal night's sleep consists of alternating NREM and REM sleep. "REM" stands for "Rapid Eye Movement" and is the phase of sleep where the brain is the most active and the most numerous and vivid dreams occur. REM sleep is also known as paradoxical sleep. "NREM" is Non-REM.
NREM sleep is itself divided into four different stages. The combination of these five stages (four NREM, one REM) creates a cycle that lasts around ninety minutes on average and repeats several times during the night.
At the end of each cycle many people actually wake up briefly. Most people don't even remember this, however some poor sleepers may wake fully.
NREM SleepThe first stage of NREM sleep is "drifting off". It lasts ten minutes or so and is accompanied by hypnagogic imagery often confused with lucid dreams. The sleeper is still half-awake and might believe themselves to be "just resting the eyes".
NREM stage 2 represents the lightest stage of "real" sleep. The sleeper can be wakened but with more difficulty than in stage 1. Brain waves become larger and the body gradually shuts down. During NREM stage 2 sleep becomes slowly deeper until...
NREM stage 3 is the threshold to deep sleep. It usually occurs between thirty to fourty-five minutes after starting to sleep. In NREM stage 3 the sleeper is much more difficult to waken and the large, slower delta brainwaves are generated.
NREM stage 4 is "deep sleep". It is during deep sleep that the body gets its most rest and does its best repair work. A sleeper in deep sleep will be very difficult to awake - sleep walking (somnambulism) sometimes occurs during this stage.
The first deep sleep period of the night is the longest and the deepest. After the deep sleep period is over the sleeper reverses the pattern, moving back up to NREM stage 2 then entering REM sleep.