There are five stages of sleep. Scientists categorized the stages of sleep based on the characteristics of the brain and body during sleep. Stage 1,2,3, and 4, are categorized as ‘non-REM sleep’, and the fifth stage, is REM sleep. Generally, brainwave frequencies and amplitudes from an electroencephelogram (EEG) are used to differentiate the different stages of sleep, along with other biologic rhythms including eye movements (EOG) and muscle movements (EMG).
Stage 1 Sleep
Stage 1 is the lightest stage of sleep. The EEG brain frequency is slightly slower than during wake time. There is muscle tone present in the skeletal muscles. Breathing occurs at a regular rate.
Stage 2 Sleep
Stage 2 usually follow Stage 1 and represents deeper sleep. During Stage 2 sleep, the sleeper is less able to be awakened. Stage 2 sleep is characterized by ‘saw tooth waves’ and sleep spindles.
Stage 3 and 4 Sleep
Stage 3 and Stage 4 sleep are progressively deeper stages of sleep. These stages of sleep are also called ‘Slow Wave Sleep’ (SWS), or delta sleep. During SWS, the EEG shows a much slower frequency with high amplitude signals (delta waves). A sleeper in SWS is often difficult to awaken. Some studies have demonstrated that very loud noises, sometimes over 100 decibels, will not awaken some during SWS. As humans get older they spend less time in slow wave deep sleep and more time in Stage 2 sleep.
Stage 5 Sleep (REM Sleep)
Stage 5, or REM sleep, is the stage of sleep associated with dreaming. It is very different physiologically from the other stages of sleep. The EEG resembles wake time. However, the skeletal muscles are atonic, or without movement. The breathing is more erratic and irregular. The heart rate often increases. It is theorized that muscle atonia evolved in order to protect the individual from injury during sleep.
Each sleep stage serves a unique role in brain and body restoration. There are many sleep deprivation studies that have demonstrated that depriving subjects of specific sleep stages has an adverse effect on body and brain functions.