Inside the sleep cycle: REM vs Non-REM

Most people think of sleep as a single block of rest. You fall asleep, and you (hopefully) wake up refreshed. But underneath that simplicity lies an incredibly sophisticated rhythm between two distinct types of sleep: non-REM and REM.

Throughout the night, your brain cycles through these stages approximately every 90 minutes, each with its own role in physical, mental, and emotional restoration.

Understanding how this sleep cycle works can help explain why some nights leave you feeling clear-headed and energised—while others don’t, even if you were in bed for eight hours.

What Is Non-REM Sleep?

Non-REM (non-rapid eye movement) sleep typically dominates the first half of the night. It’s often referred to as “deep sleep,” and is associated with physical repair and immune system recovery.

During this stage:

1. Brain activity slows significantly
2. Breathing and heart rate become steady
3. The body focuses on tissue repair, hormone regulation, and energy restoration
4. Information learned during the day begins to stabilise into memory

Non-REM sleep helps build a strong foundation for cognitive function, immune defence, and physical performance.

What is REM Sleep?

REM (rapid eye movement) sleep usually occurs more in the second half of the night. It’s known for vivid dreaming and heightened brain activity—almost as active as when you're awake.

While you’re in REM sleep:

1. Your brain forms creative connections between ideas and memories
2. Emotional experiences from the day are processed and regulated
3. Problem-solving ability improves
4. Learning is integrated and reinforced
5. REM sleep plays a vital role in mental clarity, mood balance, and resilience.

Why both stages matter

Non-REM and REM sleep serve different functions, but they work together in a complementary rhythm. Each 90-minute cycle includes both stages, and the quality of your rest depends on how well your brain moves through those cycles.

The first part of the night focuses more on non-REM sleep, while REM sleep takes centre stage in the early morning hours. That’s why cutting your sleep short—even by an hour or two—can significantly reduce the time you spend in REM, impacting how mentally refreshed you feel the next day.

Think of non-REM as your body's repair system, and REM as your brain's creative and emotional reset. Disrupting either can have consequences. For example:

1. Lack of non-REM sleep can lead to poor memory retention and a weakened immune response
2. Disrupted REM sleep is associated with increased emotional reactivity, lower creativity, and even a shorter lifespan over time.

What disrupts the sleep cycle?

Certain habits can interfere with this natural balance, making it harder to complete full sleep cycles:

1. Late-night caffeine can delay sleep onset and reduce deep sleep
2. Alcohol may cause fragmented sleep and suppress REM
3. Inconsistent bedtimes disrupt the body’s internal clock
4. Exposure to artificial light in the evening can delay melatonin release, pushing back the start of sleep

You might not remember waking up or feel like you had a poor night’s sleep, but if your cycles are disrupted, your body and brain will still feel the effects.

Signs your cycles may be disrupted

Even if you're in bed long enough, your sleep quality might be compromised if you notice:

1. Waking up feeling groggy or emotionally flat
2. Struggling with memory or concentration
3. Feeling moody or reactive
4. Needing caffeine first thing to function
5. Vivid dreams after nights of poor sleep (a sign of REM rebound)

These symptoms can signal that you're not getting enough of one sleep stage—especially REM sleep, which is crucial for mental clarity and emotional regulation.

Up next in our sleep series:

Light, melatonin & your body clock — how modern lighting affects your ability to fall and stay asleep.

Resources for deeper reading:

1. Why We Sleep by Matthew Walker (book)
2. Matthew Walker’s MasterClass on Sleep
3. Walker’s TED Talk: “Sleep is your superpower”

Want more practical tips for better rest?

Explore our Simple Sleep Guide or sign up by finding out your Sleep Chronotope, and receive the full series straight to your inbox.