Understanding Sleep Stages

Sleep is typically divided into four stages, with the first three categorized as non-rapid eye movement (NREM) sleep.

The final stage is known as rapid eye movement (REM) sleep.

A usual night involves cycling through these stages.

Stage 1

This first stage happens in the first few minutes after we fall asleep. At this point, we aren’t really in deep sleep. We’re still sort of aware of what’s around us. Our muscles begin to relax, the heart rate slows, and brain activity shifts to irregular but quick theta waves.

During this stage, it’s possible to wake up feeling like we haven’t slept at all.

After about five minutes, we transition into Stage 2.

Stage 2

Here, we’ve settled into sleep, and if we were to wake up, we’d realize we had been resting. It’s still relatively easy to awaken during this time.

This stage features brief bursts of electrical activity in the brain, called spindles, along with larger waves known as K-complexes. These signals mean the brain is still tuned into its surroundings before slipping into a more subconscious state.

Our heartbeat and breathing slow down further, and muscle relaxation deepens. Body temperature falls, and eye movements stop. Brain activity decreases, but those quick bursts of electrical signals remain present.

Stage 3

This deep sleep, or Stage 3 NREM, is crucial for waking up feeling refreshed. It usually occurs in longer spans during the earlier part of the night.

During this stage, heart rate and breathing reach their lowest levels. Brain waves become even slower, and waking someone in this stage can be quite challenging.

Meanwhile, the body engages in various restorative processes, such as repairing muscles, fueling growth, enhancing immune function, and preparing energy reserves for the day ahead.

Hypnagogia, the transitional phase between wakefulness and sleep, is linked to the first three NREM stages and can involve experiences like lucid thoughts, vivid dreams, hallucinations, and even sleep paralysis.

REM Sleep

REM sleep kicks in roughly 90 minutes after we first fall asleep.

During this phase, our eyes move rapidly behind closed eyelids, and brain wave patterns start resembling those seen when we are awake.

Breathing speeds up and becomes irregular while heart rate and blood pressure increase, reaching levels close to those of wakefulness.

Most dreaming happens during REM sleep, but there can be dreams in non-REM stages too.

Interestingly, our arm and leg muscles become temporarily paralyzed, which helps us avoid acting out our dreams. Over time, as we age, we tend to spend less time in REM sleep.

It’s thought that both non-REM and REM sleep are important for memory consolidation.

Source: US National Institutes of Health