Different Stages of Sleep

Contrary to some anecdotal information, all mammals require sleep. This sleep need is remarkably standardized in both quality and quantity and, if disturbed, results in problems during wakefulness. Sleep is divided into two distinct states known as non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). These two states occur in a roughly 90 minute cycle which is repeated 5 to 6 times a night and include four stages of NREM and REM. NREM sleep is the sleep from which REM sleep emerges (except in special cases such as Narcolepsy). As we fall asleep, we enter the transition sleep called stage 1 and begin our first "sleep cycle". Within a few minutes we evolve into our "baseline" sleep called stage 2. It is from stage 2 sleep that the other three stages emerge. Stage 2 sleep occupies approximately 50-65% of our sleep time. Within 15-20 minutes we have slowly evolved into stage 3 then stage 4 sleep, often called delta sleep or slow wave sleep because of the very high voltage, slow brain waves. Delta sleep is similar to being in a coma, but unlike a coma, it's reversible. As we transverse these first four stages of sleep our respiration and heart rate slow and the body is almost immobile. All of a sudden, after 20-30 minutes of slow wave sleep, we lighten into stage 2 and almost immediately change gears into very active brain wave pattern known as paradoxical or REM sleep. Simultaneous with this dive into REM, our respiration and heart rate increases substantially and we loose our ability to use our postural or skeletal muscles.

Along with this, our brain becomes so activated that we start to hallucinate and have what we call dreams. Our eyes move down to midline, just as in wakefulness, and they begin the move sporadically, many times in relation to what we are dreaming. In effect, we are a highly activated brain in a paralyzed body. This paradoxical state will last 10-20 minutes and then we "fall" back down into stage 2 again. This is the end of a sleep cycle and then it all starts over again, except we gradually loose our delta sleep and replace it with longer and longer periods of alternating stage 2 and REM sleep. By the final sleep cycle of the night, we will spend approximately half our time in stage 2 and half in REM. Unknown to most, this is what we call sleep in the Sleep Lab.

	Awake Low-voltage, high-frequency beta waves
	Drowsy Alpha waves prominent
	Stage 1 Sleep Theta waves prominent
	Stage 2 Sleep Sleep spindles and mixed EEG activity
	Slow wave sleep (stage 3 and stage 4 sleep) Progressively more delta waves (stage 4 shown)
	REM sleep Low-voltage, high-frequency waves

Stage 1 sleep
Stage 1 sleep is experienced as falling to sleep and is a transition stage between wake and sleep. It usually lasts between 1 and 5 minutes and occupies approximately 2-5 % of a normal night of sleep. This stage is dramatically increased in some insomnia (restless legs) and disorders that produce frequent arousals such as apnea .

Stage 2 sleep
Stage 2 sleep follows stage 1 sleep and is the "baseline" of sleep. This stage is part of the 90 minute cycle and occupies approximately 45-60% of sleep.

Stage 3 and 4 or Delta sleep
Stage 2 sleep evolves into "Delta" sleep or "slow wave" sleep in approximately 10-20 minutes and may last 15-30 minutes. It is called "slow wave" sleep because brain activity slows down dramatically from the "theta" rhythm of stage 2 to a much slower rhythm of 1 to 2 cycles per second called "delta" and the height or amplitude of the waves increases dramatically. In most adults these two stages are completed within the first two 90 minute sleep cycles or within the first three hours of sleep. Contrary to popular belief, it is delta sleep that is the "deepest" stage of sleep (not REM) and the most restorative. It is delta sleep that a sleep deprived person's brain craves the first and foremost. In children, delta sleep can occupy up to 40% of all sleep time and this is what makes children unwakeable or "dead asleep" during most of the night.

Stage 5 sleep
REM (Rapid Eye Movement Sleep): a very active stage of sleep. Composes 20-25 % of a normal nights sleep. Breathing , heart rate and brain wave activity quicken. Vivid Dreams can occur. Sleep Specialists call this 5th stage of sleep "REM" rapid eye movement sleep because if one is to watch a person in this stage, their eyes are moving rapidly about. After REM stage, the body usually returns to stage 2 sleep.

1.Sleep by J. Allan Hobson
2. Teach Yourself to Dream by David Fontana