Sleep consists of two distinct states that rotate in cycles and reflects differing levels of brain nerve cell activity.
During a normal night's sleep, one advances through these stages about five or six times:
Non-Rapid Eye Movement Sleep (NonREM: NonREM sleep is also termed quiet sleep. NonREM is further subdivided into 4 stages of progression:
- Stage 1 (light sleep).
- Stage 2 (so-called true sleep).
- Stage 3 to 4 (deep "slow-wave" or delta sleep).
Rapid Eye Movement Sleep (REM)
REM sleep is termed as active sleep and is believed by some experts to be regulated by the circadian clock in the hypothalamus. Most vibrant dreams occur in REM sleep. REM sleep may be critical for learning and for day-to-day mood regulation. When people are sleep-deprived, their brains must work harder than when they are well rested.
The body cycles through the different sleep stages from stage 1 to REM and then begins again with stage 1. Each stage represents a distinct physical and mental state of the body during sleep. During some stages, the body is in a lighter sleep and can be awakened more easily, while others indicate a very deep sleep.
Stage 1 (Drowsiness)
Within minutes you will enter the first stage of sleeping, also known as the hypnogogic state. We drift in and out of sleep for about 5 to 10 minutes and can be awakened easily. Our eyes move very slowly and muscle activity slows.
Stage 2 (Light Sleep)
Now you are really drifting off to sleep. Our eye movements stop and our brain waves (fluctuations of electrical activity that can be measured by electrodes) become slower, with occasional bursts of rapid waves called sleep spindles. Our heart rate slows and body temperature decreases.
Stages 3 and 4 (Deep Sleep)
After about 20 minutes you are really deep asleep. Your body is very relaxed, your heartbeat is slow and regular, as well as your respiratory rate. Slow brain waves called delta waves begin to appear, interspersed with smaller, faster waves.
By Stage 4 the brain produces delta waves almost exclusively. It is very difficult to wake someone during stages 3 and 4, which together are called deep sleep. There is no eye movement or muscle activity.
People awakened during deep sleep do not adjust immediately and often feel unsteady and disoriented for several minutes after they wake up. Some children experience bedwetting, night terrors, or sleepwalking during these stages.
Stage 5 - REM Sleep
REM sleep is the most interesting of the sleep stages, because from your brain activity one would say you are wide awake. During REM sleep, our breathing becomes more rapid, irregular, and shallow, our eyes jerk rapidly in various directions, and our limb muscles become temporarily paralyzed.
Our heart rate increases, our blood pressure rises, and males develop penile erections. It is in this stage that most dreaming takes place.
The average extent of time for a complete sleep cycle is 90-110 minutes. About 50 percent of sleep time is spent in stage 2 and about 20 percent in REM sleep. The remaining 30 percent is split among the other stages.
On average, a person will cycle through the stages 4 or 5 times in an eight-hour period. After a person falls asleep, the first REM sleep period generally happens 70-90 minutes later.