Stage 1: As we fall asleep, we enter the transition state called stage 1 and begin our first "sleep cycle". This very light sleep stage usually lasts 2-5 minutes. If sleep remains undisturbed it will progress to stage 2 sleep. Stage 1 contributes 2-5 % of normal sleep, (but increases hugely in cases of insomnia and disorders that produce frequent arousal such as apnea).
Stage 2: This is a much deeper sleep than stage 1. The brain waves go into theta mode, and lead into stages 3 and 4 in around 10-20 minutes. Stage 2 sleep occupies approximately 50-65% of our sleep time, lasting 15-30 minutes in each cycle. During the latter part of the night we spend more and more time alternating between stages 2 & REM sleep.
Stage 3: - A deeper sleep than stage 2. The muscles are relaxed, heart rate slows down, blood pressure falls, and breathing is steady. Brain activity slows down dramatically from the theta pattern 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.
Stage 4: - The deepest sleep of all, during which a sleeping person is 'dead to the world'. Blood pressure and heart rate fluctuate and the sleeper's brain heats up. Delta sleep is characterized by very high voltage slow brain waves. It 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.
Stage 5: - REM (rapid eye movement). All of a sudden, after 20-30 minutes of slow wave sleep, we transfer back into stage 2, and almost immediately change gears into very active brain wave pattern known as REM sleep. Simultaneous with this transfer into REM, our respiration and heart rate increases substantially and we lose our ability to use our postural or skeletal muscle. The first REM period lasts only about ten minutes. After that, the sleeper goes back into a deep stage 4 sleep. Again, the sleeper returns into a REM stage after a short period, and cycles through REM and stage 4 continue until the sleeper awakens. Along with this, our brain becomes so activated that we start to hallucinate and dream. Our eyes move as they do in our waking state, and relative to what we are dreaming. In effect, we are at this stage, 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.