What is sleep?
Sleep is a natural, reversible, repetitive state of rest in which there is altered consciousness. Sleep is mainly classified into 2 phases/ stages called NREM &REM.
- NREM-Non rapid eye movement sleep
- REM- Rapid eye movement sleep
NREM and REM stages of sleep constitute the macrostructure of sleep. This post primarily focuses on the stages of sleep and what happens during NREM, its substages and REM stages.
What happens during sleep?
- The person becomes relatively inactive and unaware of the surroundings.
- Decreased response to external stimuli.
- Reduced physical and mental activity.
NREM- Stage 1/N1 (lightest sleep)
- This stage refers to the transition period from wakefulness to sleep.
- Lasts around 5-10 minutes.
- Both alpha (initial stage of N1) and theta waves (later stage) are seen in this stage.
- A person in this stage can be easily awoken.
N2 (light sleep)/ NREM- Stage 2
- This is the longest phase of sleep (nearly half of the sleep cycle).
- The blood pressure, heart rate, respiratory rate begins to reduce.
- Body temperature reduces.
- The brain starts to produce sleep spindles.
- There are bursts of activity in the brain (K complexes) along with the sleep spindles.
- The person begins to be less aware of the surroundings.
- N2 stage lasts around 20 minutes per cycle.
- The brain collects memory and prepares for stage 3.
NREM- Stage 3/ N3 (deep sleep)
- Also called as delta sleep or slow wave sleep.
- The respiratory rate and blood pressure drops further.
- The muscles relax further.
- The brain emits delta waves or slow brain waves in this stage.
- The person is in deep sleep during this stage and it is difficult to wake the person.
- This stage lasts around 20-40 minutes.
- Sleepwalking usually occurs in this stage.
- Physical repair occurs (body repair, restore and reset) in this stage and immunity strengthens.
- The brain consolidates anything learned (declarative memory)
REM/Rapid eye movement sleep
- REM constitutes around 25% of sleep and it comprises 2 states- Phasic and Tonic.
- The brain is active and the body is relaxed.
- Dreams occur and eyes move rapidly (bursts of eye movement).
- Although voluntary muscles become immobilised/ paralysed to prevent any movement during dreaming, muscle twitches occur. Atonia occurs except in muscles of the eye and muscles that control breathing.
- REM stage occurs 90 minutes after falling asleep in normal circumstances.
- Restoration of emotional memory occurs.
- The brain cements information into memory.
- The body’s temperature is not regulated during REM sleep and hence REM sleep time is relatively shorter in cold temperatures
- Breathing and heart rate varies in this stage.
- REM is essential for cognitive functions- memory, learning, creativity etc.
The sleep cycle
- Each sleep cycle lasts for 90-120 minutes. Normally, this cycle repeats for 4-5 times (8hours).
- In the first cycle, the REM sleep occurs for 10 minutes. In the final cycle, REM lasts for 30-60 minutes.
- In the first half of the night, stage 3 NREM lasts around 20-40 minutes and as sleep progresses NREM/ deep sleep stages become shorter.
Sequence of sleep stages
The sleep cycle typically progresses in the following manner in case of a 8 hour sleep in a healthy individual
1st cycle: N1-N2-N3-N2-REM
2nd cycle: N2-N3-N2-REM
3rd cycle: Wakes briefly-N1-N2-N3-N2-REM
4th cycle:N1-N2-wakes briefly
5th cycle:N1-N2-REM-N2
Written by Auxi Arobana. R