REM and NREM Sleep Cycle - Know What Happens While You Sleep

REM sleep means Rapid Eye Movement sleep. When a person sleeps, he closes his eyes and falls into a trance of a heightened anabolic state. The sleep he enjoys, may or may not contain dreams. However, if this sleeping person encounters elaborate dreams with frequent twitching of hands and feet, and persistent movement of the eyes with a decreased ability to react to a stimulus, the term coined for such a sleep is REM sleep. A normal sleep is divided into two stages of sleep:

NREM Sleep (Non rapid eye movement sleep)
REM Sleep (Rapid eye movement sleep)

Usually, an REM cycle exhibits symptoms of irregular breathing, twitching eye movements, high blood pressure, and a sudden loss of muscle tone. The timing of the sleep cycle is associated with dreams, that take over the sleeping person for that particular duration of time. The REM cycle is also known as a partial wakefulness state constituting 20-25% of a complete sleep cycle.

Sleep Cycles During REM

A normal sleep is divided into different stages consisting of the REM and the NREM sleep cycle. Every stage of a sleep cycle is determined by the amplitude and frequency of the brain waves produced during sleep. A complete sleep cycle is about 90-100 minutes long and a normal sleep comprises 4-5 complete sleep cycles. A sleep cycle has about 5 stages, of which the first 4 sleep cycles cover the NREM sleep stage. The NREM cycles complete once, and quickly reverse again to be followed by the REM cycle. Considering that every sleep cycle is taking 100 minutes to wind, the length is approximately 10 minutes. However, this is not a fixed duration, as the sleep cycle length starts to increase with time when the third and fourth stages of NREM start to wane. The maximum length of a sleep cycle of REM is about an hour, which happens after several rounds of sleep cycles.

Stages of Sleep	Activities	Duration
Slow wave sleep - NREM Stage 1: Drowsiness	During this state, the brain emits alpha and theta waves of high amplitude and low frequency. Eyes start feeling heavy and begin to feel drowsy. A transition state between sleep and consciousness is reached.	10-15 minutes (approx.)
Slow wave sleep - NREM Stage 2: Light Sleep	The brain waves start to peak up with theta, spindles, and k-complex waves. Eye movements come to a halt and there is a gradual decrease in the temperature of the body. Heart rate also slows down.	10-15 minutes (approx.)
Slow wave sleep - NREM Stage 3: Deep Sleep	This stage is the entry point to deep sleep (also known as delta sleep). The brain waves slow down and emit low frequency delta waves, which now constitute about 20-50% of brain waves. It is difficult to awaken a person at this stage, as the blood flow to brain decreases and increases towards muscles to restore the physical energy that was depleted during the day. Immune system functioning is enhanced in delta sleep.	about 20 minutes (approx.)
Slow wave sleep - NREM Stage 4: Deep Sleep	This is the second stage of delta sleep and the deepest one too. The activity of delta and theta brain waves (constituting more than 50% of brain waves) is the maximum in this stage. During this state, the person enters a state of absolute unconsciousness. This second stage of delta sleep is the duration just before the REM sleep period commences. However, before the REM cycle starts, the first four NREM sleep cycles recur once again.	About 30 minutes (approx.)
REM sleep - Stage 5: Dream Sleep	REM sleep cycle is entered almost after 70-90 minutes of the complete sleep cycle. This stage of sleep cycle is dominated by a high frequency beta waves, that keep the brain active during sleep. Sudden muscular twitches, rapid breathing, irregular heartbeats, rise and fall in the blood pressure, penile erections (in men) and clitoral enlargement (in women) are clear signs of this sleep cycle. With high hormonal surges and active brain waves, the person encounters most vivid and realistic dreams during this stage. The psychology of dreams of the person depends on the activities he has performed in the entire day. There can be 3-5 REM episodes for a person in his sleep cycle.	10 minutes to 1 hour

Sleep is the most wonderful experience cum routine for a person. It is a natural form of meditation with absolute concentration. Deprivation of a regular sleep routine can lead to impaired thought processes, weakened immune system, chronic depression, and sleep disorders like insomnia. The significance of REM sleep cycle is that, it keeps the brain active and enhances the cognitive thought processes of the person by choosing to remember what is required and discard what is not required.