What Are the Stages of Sleep and Why Do They Matter?

Have you ever wondered why the recommendation is eight hours of sleep? It breaks down nicely for most people. A third of the day sleeping, a third of the day working and a third of the day enjoying yourself … or at least attempting to.

But enjoyment is hard to come by if you’re dragging due to a lack of quality sleep.

We’ve landed on roughly eight hours as a society not just because it neatly fits into our 24-hour days, but because most studies indicate its usefulness. In 2015, the National Sleep Foundation in the US released updated recommendations for sleep durations based on scientifically sound and practical understandings for daily sleep duration across all different age groups.

sleep recommendations by age

But there is no exact “magic number” for hours of sleep for everyone universally. Your body is unique and its needs are only its needs. 

The important thing to understand is why do we need sleep, what happens when we sleep and how can we create good sleep habits to ensure we’re maximizing our time.

What Are the Sleep Stages?

Sleep consists of several stages that cycle throughout the night in a predictable pattern.

The two main categories of sleep are non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. To build good sleep habits and understand why you’re doing so, it’s helpful to understand not only what the stages of sleep are, but what’s going on with your body and brain during each stage.

Stage NREM-1: This is the transitional stage between wakefulness and sleep. It is a light sleep stage during which you may experience hypnic jerks (sudden muscle contractions) or feel like you are drifting in and out of sleep. In this stage, things are delicate and a big movement from your dog or partner can result in you being awake for another hour or so. 

In this stage, brain waves slow down, and you may have some fleeting thoughts or images. Brain waves are tiny little electrical voltages in the brain - yes, your brain generates a very, very small bit of electricity. There are four types of waves, alpha, beta, theta and gamma waves and each is associated with a different bit of activity.

During stage NREM-1 sleep, alpha and theta brain waves, relatively low-frequency patterns, are active. Among those, theta waves are most dominant during this transitional stage. Theta brain waves are associated with a state of relaxation, calm and drowsiness. Hence the gentle fade into sleep so many people experience.

In your brain, the neurotransmitter acetylcholine is active during this stage, slowly decreasing in levels, contributing to the transition from wakefulness to sleep. Acetylcholine works to increase your alertness throughout the day.

Stage NREM-2: This is a deeper sleep stage in which brain activity further slows down, and sleep spindles (bursts of rapid brain activity) and K-complexes (large and slowly occurring delta brain waves) may occur. This stage makes up a significant portion of total sleep time. 

The spindles inside your brain are strongly correlated with the twitching feeling you may be familiar with when falling asleep. K-complexes are longer and slower waves lasting a full second and work to keep you asleep and unbothered by any outer disturbances.

Even when we’re moving into a deeper sleep, the brain still works hard! Give your brain some credit today if you haven’t yet.

While acetylcholine levels continue to decrease, other neurotransmitters like serotonin and norepinephrine decrease as well, contributing to the suppression of arousal and wakefulness. The decrease of each of these neurotransmitters associated with you being awake and getting things done all plays a big role in you moving into a state of sleep where it’s harder to wake up. 

That’s one of the reasons you don’t want to end up in a deep stage of sleep when you’re just looking for a quick nap. Waking here leaves you groggy.

Stage NREM-3 (formerly Stage NREM-3 and Stage 4): This is the deepest and most restorative stage of sleep. It is also called slow-wave sleep (SWS) due to the slow delta brain waves that dominate this stage.This deep sleep state allows the body to focus on essential physiological processes such as tissue repair, muscle growth, and immune system strengthening.

The brain basically is going to the gym every night.

During this deep sleep stage, there is an even further reduction in acetylcholine, serotonin, and norepinephrine. Instead, gamma-aminobutyric acid (GABA) becomes more active, which helps to inhibit brain activity and promote deep sleep.

Growth hormone release is prominent during NREM-3, supporting tissue repair, muscle growth, and overall physical restoration.

Waking up during stage NREM-3 is the worst. You’ll need the most time for your brain to transition out of sleep and into real life once again.

Stage REM: The last stage is perhaps the most famous: Rapid Eye Movement (REM) sleep.

As expected, this stage is characterized by rapid eye movements, increased brain activity, and vivid dreams. This stage is crucial for cognitive functions, learning, and emotional regulation. Your body becomes temporarily paralyzed during REM sleep, likely to prevent you from acting out your dreams.

During REM sleep, the brainwaves resemble the wakeful state, with increased activity and faster frequencies. The brain exhibits beta and gamma waves, which are similar to those seen during active wakefulness. Additionally, there is a suppression of muscle activity during REM sleep, which is called muscle atonia.

After reducing in each previous stage, acetylcholine levels increase significantly during REM sleep, leading to vivid dreaming and the activation of brain regions associated with memory and emotion. Other neurotransmitters, such as serotonin and norepinephrine, remain at low levels, contributing to muscle atonia (temporary paralysis).

During REM sleep, the body exhibits fluctuations in cortisol (stress hormone) and sex hormones like testosterone. Additionally, there may be increased secretion of neurotransmitters involved in emotional regulation, such as dopamine and serotonin.

Throughout the night, the sleep stages cycle through several times, with REM periods typically getting longer in the later part of the night. Each full cycle lasts roughly 90 to 120 minutes, and a typical night's sleep consists of 4-6 cycles. But you don't spend an equal amount of time in each cycle.

time spent in sleep stages

It's essential to go through all these stages a number of times for a good night's rest. And that brings us back to our original point: eight hours of sleep. Getting seven, eight or nine hours of sleep allows you to get into your deepest and more restorative sleep stages often enough to give your body the recuperation and rest it always needs.