The different stages of the nightly sleep cycle are shown in the figure. Most dreaming occurs during REM sleep (marked in red), although some can occur outside of REM sleep.
Sleep is typically divided into four stages. The first three of these are known as “non-rapid eye movement” or NREM sleep.
The last stage is known as rapid eye movement or REM sleep.
A typical night’s sleep goes back and forth between stages.
Stage 1 : For about the first five minutes after you fall asleep, we are not in a deep sleep.
We are still aware of our surroundings, but our muscles begin to relax, our heartbeat slows down, and our brain wave patterns, known as theta waves, become irregular but rapid.
Even though we are asleep during stage 1, we may wake up feeling like we didn’t sleep at all.
After about five minutes, our bodies move into the second stage.
Stage 2 : This is when we have fallen into a deep sleep and if we woke up you would know we were asleep. Waking up is still quite easy.
This stage is identified by short bursts of electrical activity in the brain, known as spindles, and larger waves known as K complexes, which indicate that the brain is still aware of what is happening around it before shutting down at a subconscious level.
Heart rate and breathing slow, and muscles relax even more.
Our body temperature drops and eye movements stop.
Brain wave activity slows down, but is characterized by short bursts of electrical activity.
Stage 3 : Stage 3, which is non-REM sleep, is a period of deep sleep that we need to feel refreshed in the morning.
This occurs over a longer period of time in the first half of the night.
Our heart rate and breathing slow to their lowest levels during sleep, and brain waves become even slower.
Our muscles are relaxed and it can be difficult for people to wake us up.
The body repairs muscles and tissues, stimulates growth and development, boosts immune function, and increases energy for the next day.
Hypnagogia – the transitional state between wakefulness and sleep – is associated with NREM stages one through three.
Mental phenomena during hypnagogia include clear thought, lucid dreaming, hallucinations, and sleep paralysis.
REM sleep : REM sleep first occurs about 90 minutes after falling asleep.
Our eyes move quickly from side to side behind closed eyelids.
Mixed-frequency brain wave activity becomes closer to that seen in wakefulness.
Our breathing becomes faster and irregular, and our heart rate and blood pressure rise to almost waking levels.
Most dreaming occurs during REM sleep, although some can also occur outside of REM sleep.
The muscles in our arms and legs become temporarily paralyzed, which prevents us from acting out our dreams.
As we age, we spend less time in REM sleep.
Memory consolidation likely requires both REM and non-REM sleep.
Source: US National Institutes of Health
