Sleep progresses through various stages; these stages are characterized by different brain wave pattern. The study of sleep was only possible after the invention of electroencephalograph. Prior to its invention scientists didn’t have a means to study sleep. There are two types of sleep:
- Quiet Sleep or the Non-Rapid Eye Movement Sleep
- Active or Paradoxical Sleep or the Rapid Eye Movement Sleep
At the early stages of sleep, a person is comparatively awake and alert. There is production of beta waves in the brain which are small and fast. When the brain begins to relax even smaller waves, called alpha waves are produced. This is the stage when you are not asleep but also not wide-awake. During this time, one may experience vivid sensation known as hypnagogic hallucinations. Common examples include hearing someone calling out your name.
Another experience that is common during this phase is known as myoclonic jerk in which the person wakes up with a startle without any reason at all. As unusual as it may sound, a monoclonic jerk is actually a common feeling that occurs in many individuals.
Stage 1 of Sleep:
This is just the beginning of the sleep cycle. Hence, the intensity of sleep is quite light and it is easy to wake up a person from such sleep. This period is a transition stage between wakefulness and sleep. In this stage the brain produces the theta waves that are very slow brain waves. This stage lasts for about 5 to 10 minutes.
Stage 2 of Sleep:
In the second stage of sleep that lasts for around 20 minutes, the brain produces a rhythmic brain wave activity known as sleep spindles. The temperature of the body decreases and the heart rate slows down.
Stage 3 of Sleep:
In this stage, slow brain waves called delta waves starts to emerge. This is the stage when the sleep starts transitioning from light sleep to very deep sleep.
Stage 4 of Sleep:
Also referred to as delta sleepthe stage 4 of sleep lasts for about 30 minutes. This is a deep sleep stage and is the last phase of non-rapid eye movement (NREM) sleep.
Stage 5 of Sleep:
Stage 5 is the rapid eye movement (REM) stage in which all the dreams occur. This stage is characterized by eye movement, increased brain activity and heightened respiration rate. This stage is also known as paradoxical stage because in this stage the brain is active while the body muscles are relaxed. In general, a person enters the REM stage of sleep after 90 minutes of falling asleep.
The Sequence of Sleep Stages
So, these were the stages of sleep. But, it does not necessarily mean that sleep follows these sleep stages in sequence. Sleep typically begins with the stage 1 and eventually progresses to 2, 3 and 4, after which it goes back to stage 3 and stage 2 and after that it enters the stage 5 which is the REM stage of sleep. Once the REM sleep ends, the sleep returns to stage 2. In general, a person passes through the various stages of sleep about four to five times throughout the night. The first stage of REM sleep is lasts for a short time, but with each successive cycle, it lengthens and can last up to an hour.