When you fall asleep, the pattern of sleep is not uniform throughout. There are variations in the sleep patterns and these variations constitute the sleep cycle. These sleep variations also form different stages of sleep and define how you fall asleep.

Each stage of sleep of the sleep cycle is different from each other in terms of what happens in the brain and body. Each part of the sleep has a different function to perform in preparing you for the next day and it is important that you spend appropriate sleeping hours in each stage of sleep.

How is a sleep study done?

Polysomnography, a sleep study is done to diagnose sleep disorders. It does this by recording your sleep stages during your sleep cycle when you are asleep.

Electroencephalogram (EEG) is a test to study the electrical activity of the brain. When it is recorded during sleep, it is not uniform throughout your sleep and it changes over the course of the night. It is these changes, which help to chart out the various stages of sleep.

Studies to know what happens when you fall asleep were conducted using the electroencephalogram (for brain activity), electrooculogram (for eye movements) and electromyogram (for muscle activity).

Stages of Sleep

These studies revealed that there are two main stages of sleep. These stages were defined by the electric patterns in the brain and the presence and absence of eyeball movements. The two types or stages of sleep are:

1)      Non-rapid eye movement (NREM) sleep is also called quiet sleep. This stage of sleep consists of four sub-stages each being deeper than the previous one. N1, N2, N3 and the 4th stage of NREM sleep, which is also known as the delta sleep or slow-wave sleep or the stage of deep sleep. In NREM sleep, as the stages of sleep progress, the brain waves become slower and more synchronized.

2)      Rapid Eye Movement (REM) sleep is also called active sleep or paradoxical sleep. This is the part of sleep when you actively dream and the eyeballs move back and forth, which is why it is called rapid eye movement sleep.

Let us simplify this:

  • Stages 1 and 2 comprise of light sleep
  • Stages 3 and 4 bring deep sleep
  • Stage 5 is the part of sleep in which the eyeballs are closed but move rapidly from side to side due to intense brain activity. This is the stage during which you dream.

Sleep Cycle

The sleep cycle consists of the four stages of NREM sleep and one of REM sleep. It is therefore also called the REM-NREM cycle. It is a fluctuation between the slow-wave or deep sleep phase and the REM phase of sleep. 

In stage one, you sleep lightly and you float in and out of sleep. Your eye movement is slow and muscle activity weak. It is easier to wake up during this stage of sleep.

The second stage prepares your body for deep sleep. Your eye movements, muscle activity, and the brain waves slow down, your body temperature and heart rate drop.

You move into the third stage in deep sleep. Your brain waves (delta waves) are extremely slow but intermixed with smaller but faster brain waves.

In stage four, you are in deep sleep and the brain is now producing only the slow delta waves.

The last fifth stage is called the stage of active or paradoxical sleep (REM sleep). Though your eyes are closed, they are rapidly moving from side to side because you dream intensely and your brain activity increases.

In humans, each sleep cycle typically lasts for 90 to 110 minutes and then begins again. About four to six cycles are repeated over the night’s sleep.

When you fall asleep, the sequence of the sleep stages in a sleep cycle does not follow the same order.

What Happens When You Fall Asleep? Sequence of the sleep stages

Sleep does not occur in the sequence mentioned above. Sleep begins with stage one (N1) and progresses to stage N2, N3 and then to the stage of delta sleep.

After this stage, N3 and then stage N2 (of 5 to 10 minutes) are repeated before the onset of the REM stage. After the REM stage, the body returns to stage N2.

Scientists are not sure but speculate that this specific sequence of NREM and REM sleep occurs to optimize the mental and physical recuperation and certain aspects of memory consolidation.

Causes of changing sleep patterns in the sleep cycle

Various factors influence shifting sleep patterns. They are

  • Age: In an infant, sleep often begins with REM sleep and the sleep cycle is much shorter at 50 to 60 minutes. The average length increases as the baby grows to become an adult.
  • Amount of recent sleep or wakefulness
  • Other influencing factors occurring before going to sleep such as stress, exercise
  • Environmental conditions such as climate and light

Sleep history of an individual also can shift sleeping patterns. Missing a night’s sleep, irregular sleeping routines and frequent disturbance of sleep patterns can alter the sequence of the sleep cycle.

Some drugs too can alter the sleep cycle events and adversely influence how you fall asleep. Alcohol taken before sleep tends to suppress the REM sleep early in the night. After the alcohol is metabolized late in the night, REM sleep tends to rebound.

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