The Sleep Cycle: WTF is REM Sleep anyway?
Sleep: we spend about a third of our lives doing it, yet many of us know very little about what actually happens during those hours. Your brain and body are hard at work cycling through distinct stages, each with its own unique purpose. Let’s take a closer look at the science of sleep and break down the two major phases: NREM (Non-Rapid Eye Movement) and REM (Rapid Eye Movement) sleep.
What Is the Sleep Cycle?
The sleep cycle refers to the recurring pattern of NREM and REM sleep that your body moves through multiple times each night. A single cycle typically lasts about 90 minutes, and most people complete four to six cycles per night. The balance of NREM and REM shifts as the night progresses, with NREM dominating the earlier cycles and REM becoming more prominent toward morning.
Each phase plays a vital role in repairing your body, consolidating memories, and regulating emotions. Miss out on either, and you’re likely to feel it the next day—whether it’s in the form of physical fatigue, mental fog, or emotional instability.
The stages of a typical night’s sleep. Notice how REM sleep becomes more concentrated in the later part of the night, right up until morning.
The NREM Phase: Your Body’s Repair Shop
NREM sleep accounts for about 75% of the sleep cycle and is divided into three stages (formerly four, but scientists now combine two of the stages). This phase is all about physical recovery and restoration.
Stage 1: The Gateway to Sleep
Stage 1 is the lightest stage of sleep, lasting only a few minutes. During this time:
Your heart rate and breathing slow down.
Your muscles begin to relax, sometimes causing the infamous hypnic jerk (that sudden feeling of falling that always seems to happen when you’re sleeping in public).
Brain activity shifts from wakefulness to theta waves, which are slower and more rhythmic.
Stage 1 is essentially the on-ramp to deeper sleep, where your body starts to disconnect from its surroundings.
Stage 2: Light Sleep
This stage typically lasts 10-25 minutes in the first cycle and gets longer with each subsequent cycle. During Stage 2:
Your body temperature drops, and your muscles relax further.
Brain activity features sleep spindles (bursts of rapid brainwave activity) and K-complexes, which are thought to protect your sleep from external disturbances.
The body begins its repair processes, and heart rate slows further.
Stage 3: Deep Sleep (Slow-Wave Sleep)
Also called slow-wave sleep (SWS), this is the most restorative stage of NREM sleep. It dominates the earlier part of the night and is characterised by:
Delta Waves: The slowest and most synchronised brain activity.
Physical Repair: Growth hormone is released, aiding in cell repair, muscle recovery, and immune function.
Energy Restoration: The body replenishes energy stores and clears metabolic waste from the brain via the glymphatic system.
Deep sleep is critical for physical health. It’s what leaves you feeling refreshed and rejuvenated the next morning. Without enough deep sleep, you may experience muscle fatigue, weakened immunity, and difficulty concentrating.
The REM Phase: The Mind’s Playground
While NREM sleep is all about physical recovery, REM sleep focuses on the brain. Accounting for about 20-25% of total sleep, REM is the most fascinating and mysterious phase.
What Happens During REM Sleep?
Brain Activity: REM sleep features brain activity similar to wakefulness, with rapid, desynchronised brain waves. This is why it’s often called “paradoxical sleep”—your brain is highly active, but your body is deeply relaxed.
Dreaming: Most vivid dreams occur during REM, as the brain processes emotions, experiences, and memories.
Muscle Paralysis: To prevent you from acting out your dreams, your body experiences atonia, a temporary paralysis of voluntary muscles. Atonia is not to be confused with sleep paralysis which is a temporary inability to move or speak, accompanied by a conscious awareness of being immobilised. It occurs when your brain wakes up from REM sleep, but your body remains in a state of atonia. This disconnect creates the sensation of being paralysed. No likey.
Eye Movement: REM gets its name from the rapid, darting movements of your eyes beneath your eyelids.
Why Is REM Sleep Important?
Memory Consolidation: REM sleep strengthens neural connections, helping your brain retain information and transfer short-term memories into long-term storage.
Emotional Regulation: REM sleep allows you to process emotions and recover from stress, which is why poor REM sleep is linked to irritability and mood disorders.
Creativity and Problem-Solving: By integrating disparate ideas and experiences, REM sleep fuels creativity and innovative thinking.
Interestingly, the amount of REM sleep increases as the night goes on. That’s why cutting your sleep short—even by an hour—can disproportionately affect your mental clarity and emotional balance the next day.
NREM vs. REM: A Symbiotic Relationship
NREM and REM are not competing forces but complementary processes. NREM is essential for rebuilding the body, while REM ensures the mind is sharp and ready to tackle the challenges of the day. Here’s how they work together:
Early in the night, you’ll spend more time in deep NREM sleep to prioritise physical recovery.
As the night progresses, REM phases lengthen to focus on mental and emotional health. During REM it’s easier to be disturbed and woken up.
Both phases are equally important, and disrupting the balance can lead to significant health consequences. Chronic sleep deprivation affects everything from immune function to memory, mood, and even longevity.
How to Optimise Your Sleep Cycle
Understanding the science of sleep is the first step; the next is creating habits that support a full night of high-quality sleep. Here are some tips:
Stick to a Schedule: Go to bed and wake up at the same time every day to regulate your circadian rhythm.
Create a Sleep-Friendly Environment: Keep your bedroom cool, dark, and quiet—like a cave. Use blackout curtains or an eye mask.
Limit Stimulants: Avoid caffeine at least 10 hours before bed. Caffeine won’t stop you falling asleep, but will prevent you from falling into deep sleep.
Dim the Lights: While scientific research on blue light's precise effects remains mixed, reducing screen brightness and dimming household lights before bedtime can help create a more relaxing environment that signals to your body it's time to wind down. And that’s no bad thing.
Stay Active: Regular exercise promotes deeper NREM sleep and more consistent REM cycles—just don’t work out, or have a hot shower, too close to bedtime. In the run up to sleep, your body’s temperature starts to fall and this is linked to a reduction in the time it takes to fall asleep.
Mind Your Diet: Eat a balanced diet and avoid heavy meals close to sleep.
Final Thoughts
Your sleep cycle is the cornerstone of your overall health and well-being. By better understanding and respecting the intricate dance between NREM and REM sleep, you can set the stage for nights of truly restorative rest.
We hope some knowledge can guide you, but don’t underestimate the power of a consistent bedtime routine—even on the weekends. Changing into sleepwear also helps signal to the brain it’s time to sleep, and can help minimise any disturbances, especially during the change from one sleep cycle to the next.
Sleep better. Be better.
