Sleep Stages

Although sleep may seem like and once considered as a steady state, but it is actually have several stages that cycle throughout the entire night. The brain wave (amplitudes and frequencies) present at a particular time determine the stage of sleep. Most important sleep stage (5th stage) occurs with lucid dreams, called as the Rapid Eye Moment (REM) stage. REM sleep is characterized by rapid eye movement while dreaming; dreaming occurs even during the other sleep stages, but the most vivid dreaming only happens in REM stage.

REM & NREM
Sleep follows a certain regular sleep cycle every night. There are two basic forms of sleep: rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep. REM sleep is sometimes called as paradoxical sleep. Infants have about half of their sleep time in REM sleep. Adults have approximately 20% of their sleep time in REM and balance NREM sleep. Elderly people have less than 15% of their sleep time in REM sleep balance NREM sleep.

Sleep cycle
One complete sleep cycle takes about 90 to 100 minutes, thus for an average sleep time of  8 hours there will be a 4 to 5 complete sleep cycles. The sleep cycle begins with four stages of sleep called by Slow Wave Sleep (SWS) or non rapid eye movement (NREM or Non-REM) sleep. After the completion of the first 4th stages, instead of proceed to 5th REM sleep the first 4 stages quickly reversed and then followed by REM sleep. The first REM sleep will occur approximately after 90 minutes of falling asleep and it will last only about 10 minutes, gives the starting length of sleep cycle being approximately 100 minutes. The length of the stages of sleep is not fixed and it varies, in particular the length of sleep stage 3 and 4 (also called delta or deep sleep) slowly wane, and the length of stage 5 (REM sleep) increases, up to about one hour in length after certain sleep cycles. Therefore, as the night proceeds, dream may be of longer periods of time.

Summary of the sleep pattern – One sleep cycle

Sleep stage 1 is the light sleep where there is a drift in and out of sleep and one can be awakened easily during this stage. This stage is characterized by slow eyes movement and slow muscle activity. During this stage, many people experience sudden muscle contractions preceded by a sensation of falling.

Common characteristics of stage 1

• transition between sleep and wakefulness
• eyes rolls slightly
• mostly contains of theta waves (high amplitude & low frequency – slow wave)
• briefly contains of alpha waves, similar to those present while awake
• Stage 1 lasts only for a few minutes

Sleep stage 2 with eye movement stopped and the brain waves become slower with only an occasional burst of rapid brain waves.

Common characteristics of stage 2

• Amplitude of brain wave peaks become higher and higher (sleep spindles)
• k-complexes (peaks suddenly descends and then rises) follow spindles
• Stage 2 lasts for a few minutes

Sleep stage 3 with extremely slow brain waves called delta waves are interspersed with smaller, faster waves. This stage is considered as deep sleep and so it is difficult to waken someone from this stage.

Common characteristics of stage 3

• Called as delta sleep or deep sleep
• very slow brain waves, called delta waves (lower frequency than theta waves)
• 20 - 50% of brain waves are delta waves; the balance are theta waves

Sleep stage 4 with the brain produces delta waves almost exclusively. This stage is also considered as deep sleep and so it is difficult to waken someone from this stage. In deep sleep, there is no eye movement or muscle activity. This is when some children experience bedwetting, sleepwalking or night terrors.

Common characteristics of stage 4

• Called as delta sleep or deep sleep
• more than 50% of brain waves are delta waves; the balance are theta waves
• last sleep stages before REM sleep; sleep stages reverses and then REM sleep begins

Sleep stage 5 – REM stage with breathing becomes more rapid and shallow, eyes jerk rapidly and limb muscles are temporarily paralyzed. The waveform during REM has low amplitudes and high frequencies, just like the waking state. Early researchers actually called it "paradoxical sleep". REM stage has increased heart rate, raised blood pressure, males have erections and the body does not regulate its temperature. Most of the vivid dreams occur during REM sleep, once awaken at this stage a person can remember the dreams. Most people experience three to five times of REM sleep each night.

Common characteristics of stage 5

• Beta waves - have a high frequency and it happens when the brain is quite active, that is during REM sleep and while awake
• frequent bursts of rapid eye movement, along with infrequent muscular twitches
• heart beat faster with shallow and rapid breathing
• most vivid dreaming occurs during REM

Infants spent almost 50% of their sleep time in REM stage. Adults spend nearly half of sleep time in stage 2, about 20% in REM and the other 30% is divided between the other three stages. Older adults spend progressively less time in REM sleep.

Look at the differences in the EEG (shows and record of brain activity), EMG (shows and record muscle activity) and EOG (shows and record eye movements) during waking, REM sleep and NREM sleep.

During asleep, our brain waves pass through different stages of sleep. Start first from stage 1 sleep for few minutes, then proceed to stage 2 sleep, then stage 3 sleep, then stage 4 sleep. Then it's back up again in the reverse order stage 3, stage 2, then REM sleep, then it repeat down & up again as shown in the figure. In an 8 hour sleep, the brain cycles through these stages about 4-5 times.

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