The sleep process is not just a single continuous period of unconsciousness from which a person can awaken. It is a dynamic and active process with various stages of sleep alternating through a single sleep cycle. There are broadly two stages of sleep :
- Slow wave sleep (non-REM)
- REM sleep (Rapid Eye Movement)
There are four stages in slow-wave sleep often resulting in REM sleep being labeled as the fifth stage. Sleep related disorders, known as parasomnia, can occur in both non-REM and REM sleep. This is discussed further under types of parasomnia.
Slow-wave sleep is a period marked by very low frequency brain waves and accounts for most of the sleep cycle. It is also commonly referred to as deep sleep or non-REM sleep. Although vivid dreaming is associated with REM sleep, dreams may also occur during this stage. Most of the dreams during slow-wave sleep, however, are not remembered. Physiological changes, namely a decreases in the blood pressure, breathing rate and metabolic rate are seen in this period.
Sl0w-wave sleep is very restful and rejuvenating and occurs within the first hour to 90 minutes of sleep. Most ‘power naps’ only involve slow-wave sleep and the physiological changes that occurs during this time is the reason that a person feels invigorated after such a short rest.
This is the stage between sleep and wakefulness. A person is awakened easily during this period and stage 1 of sleep is usually not repeated again in the sleep cycle.
Sleep actually begins in this stage and commences as light sleep. Muscles relax, blood pressure and the breathing rate decreases.
This is the stage between light and very deep sleep where the first very slow delta brain waves are seen.
This stage is marked by delta brain waves and is known as very deep sleep. Stage 4 lasts longer, sometimes twice as long as any of the preceding three stages. Many non-REM sleep behavior disorders occur during this stage.
REM sleep is also known as paradoxical sleep or the dream stage. It is a period where there is rapid movement of the eyes and is associated with vivid dreaming. REM sleep is not as restful as slow-wave sleep but occurs every 90 minutes of the sleep cycle. It can last anywhere from a few minutes to 30 minutes and accounts for about a quarter of the sleep cycle. It is not uncommon for bouts of REM sleep to be absent in a sleep cycle.
The brain becomes highly active during REM sleep and the peripheral muscles are inhibited. This is due to the dream state and preventing movement during this period is important to prevent a person from enacting out dreams. Unlike the slow-wave sleep stage, the heart and respiratory rate are irregular.