What are sleep cycles and stages?

What are sleep cycles and stages?

Many people think nothing much happens when we sleep, but that's far from true. Sleep is a dynamic process where your mind and body are hard at work. Every night, you go through several sleep cycles, each with different stages.

Key takeaways

  • Sleep is divided into cycles, each lasting about 90 minutes.
  • Each cycle is divided into stages, with four stages in total.
  • Each stage of sleep has a specific role to keep us healthy and happy.

The 4 stages of sleep

Stage 1: light sleep (doze off)

5% of total sleep time

When you first fall asleep, you enter Stage 1. Think of this as dipping your toes into the pool of sleep. Your body starts to relax, your breathing slows down, and your brain waves begin to slow. These brain waves, known as theta waves, are slower than the beta waves your brain produces when you're awake. You might still be aware of some sounds around you, but you're on the edge of dreamland [2].

Stage 2: true sleep (sleep steady)

40-55% of total sleep time

Stage 2 is when you really start sleeping. Your body relaxes more, your heart slows down, and you get a bit cooler. Your brain does something cool here - it makes quick bursts of activity called sleep spindles. These help you remember things from your day [3].

If you've ever taken a short nap and felt better after, you were probably in Stage 2 sleep. It's deep enough to help you feel rested, but not so deep that it's hard to wake up. You spend a lot of time in this stage, which is totally normal. If you use a sleep tracker, it might show this as "light sleep" along with Stage 1. 

Stage 3: deep sleep (repair stage)

10-20% of total sleep time

Welcome to Stage 3, the deepest stage of sleep. During this time, your body produces delta waves, the slowest brain waves, crucial for restorative sleep. Your body is busy repairing muscles, building bones, and strengthening your immune system. If someone tries to wake you up now, you might feel groggy and disoriented because your brain is in a slow-wave state [4].

Stage 4: REM sleep (dream stage)

20-25% of total sleep time

REM stands for Rapid Eye Movement. This is when your brain is super active, almost like it's watching an exciting movie. Your eyes move quickly under your eyelids, and you start to dream. REM sleep is vital for learning, memory, and mood regulation. It's your brain's playtime, sorting through everything you experienced during the day. During this stage, your body undergoes temporary paralysis to prevent you from acting out your dreams, keeping you safe from potential harm [5].

The cycles of sleep

Sleep Cycles

Each sleep cycle is a complete journey through various stages. Instead of staying in one stage all night, you go through multiple cycles, transitioning from light sleep to deep sleep and then to REM sleep. 

The first sleep cycle usually lasts around 70 to 100 minutes, while subsequent cycles last a bit longer, about 90 to 120 minutes. This pattern forms our sleep structure, known as sleep architecture. It's also normal to experience brief awakenings between stages, some so brief you may not even realize they happen. This sleep architecture ensures we get the right balance of all sleep stages every night [6].

Is my sleep cycle normal? 

A "normal" sleep cycle includes all stages of sleep and repeats several times throughout the night. It's not about the exact time you go to sleep, but about maintaining a regular sleep schedule and ensuring you get sufficient total sleep. On average, adults need 7-9 hours of sleep per night to experience enough cycles and stages for optimal health [7].

Why sleep architecture is important and how disruptions affect you 

When sleep architecture is disrupted by stress, illness, or poor sleep habits, we might not get enough restorative sleep, leaving us feeling tired and groggy. Long-term sleep disruptions can lead to significant health issues, including weakened immune function, impaired memory, and mood disorders [8].

For example, people with sleep apnea have their breathing repeatedly stop and start during sleep. This condition disrupts sleep cycles as the person may wake up multiple times throughout the night to resume breathing. These frequent interruptions prevent the person from reaching deep sleep and REM sleep stages, leading to excessive daytime sleepiness, cardiovascular problems, and other long-term health issues [9].

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Sleep Apnea

How do I know my sleep cycle stages? 

Various methods exist to monitor sleep cycles, ranging from wearable devices to home sleep tests and polysomnography. These methods differ in terms of effort required and precision of results [10].

To better understand sleep stages, we need to measure several key indicators.

Brain waves (EEG)

These change throughout sleep, reflecting different stages. Slower waves indicate deep sleep, while faster waves characterize REM sleep. Measuring the electrical activity in the brain helps identify these various stages of sleep.

Eye movements (EOG)

Eye movements are particularly important for identifying REM sleep, when dreaming typically occurs. During this stage, rapid eye movements occur under the eyelids, indicating high brain activity similar to processing visual information while awake.

Muscle activity (EMG)

This is another crucial indicator, especially during REM sleep. In this stage, muscles are mostly paralyzed, a phenomenon known as atonia. This paralysis serves a vital function, preventing individuals from physically acting out their dreams and ensuring safety during sleep.

Heart rate and oxygen levels 

Heart rate typically slows during non-REM sleep and may vary during REM sleep. While oxygen levels usually remain stable throughout the night, drops in oxygen saturation can indicate issues like sleep apnea, which can significantly affect overall sleep quality.

Respiratory effort and airflow

These measurements are essential for detecting sleep disorders such as sleep apnea. These indicators help assess breathing patterns throughout the night, which are crucial for evaluating overall sleep quality and identifying potential disruptions to the sleep cycle.

By measuring these indicators, clinicians can accurately identify sleep stages, diagnose potential sleep disorders, and gain a comprehensive understanding of your sleep patterns and quality.

Wearable Devices

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Smartwatches and fitness trackers, such as Oura rings, Fitbit, and Apple Watch, use sensors to estimate your sleep stages by monitoring your heart rate, movement, and other indicators. These devices offer a convenient way to track your sleep patterns at home.

However, they often lack precision. They provide a general overview of your sleep stages but can miss subtle changes and may not always accurately measure stages of sleep. 

Polysomnography (PSG)

Polysomnography

Polysomnography is the most traditional method for studying sleep. You might have seen pictures of people in sleep labs with numerous wires attached to their bodies—that’s polysomnography. It monitors brain waves, oxygen levels, heart rate, breathing patterns, and more, providing detailed information about your sleep stages and cycles.

While polysomnography is the gold standard for diagnosing sleep disorders and understanding your sleep architecture, many people find it difficult to sleep well in a lab setting, which can affect the results.

Home Sleep Tests

Today, advanced home sleep tests can be as reliable as polysomnography for measuring sleep cycles. These tests can measure various aspects of your sleep, including breathing, oxygen levels, heart rate, and even brain activity. They are more convenient and easier to use than polysomnography since you can perform them in the comfort of your own home. 

(Curious about the advantages and limitations of polysomnography versus home sleep tests? Check this article)

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That’s all for sleep cycles and stages today! If we could give you one quick tip for maintaining a healthy sleep cycle: stick to a sleep schedule. That is, go to bed and wake up at the same time every day (yes, even on weekends). This helps maintain your sleep stages consistently! 

Last updated: September 23rd, 2024

References

[1] Walker, M. (2017). Why We Sleep: Unlocking the Power of Sleep and Dreams. New York: Scribner.

[2] Carskadon, M. A., & Dement, W. C. (2011). Normal Human Sleep: An Overview. In M. H. Kryger, T. Roth, & W. C. Dement (Eds.), Principles and Practice of Sleep Medicine (5th ed., pp. 16-26). St. Louis: Elsevier Saunders.

[3] Rasch, B., & Born, J. (2013). About sleep's role in memory. Physiological Reviews, 93(2), 681-766.

[4] Xie, L., et al. (2013). Sleep drives metabolite clearance from the adult brain. Science, 342(6156), 373-377.

[5] Hobson, J. A. (2009). REM sleep and dreaming: towards a theory of protoconsciousness. Nature Reviews Neuroscience, 10(11), 803-813.

[6] Feinberg, I., & Floyd, T. C. (1979). Systematic trends across the night in human sleep cycles. Psychophysiology, 16(3), 283-291.

[7] Hirshkowitz, M., et al. (2015). National Sleep Foundation's sleep time duration recommendations: methodology and results summary. Sleep Health, 1(1), 40-43.

[8] Medic, G., Wille, M., & Hemels, M. E. (2017). Short- and long-term health consequences of sleep disruption. Nature and Science of Sleep, 9, 151-161.

[9] Malhotra, A., & White, D. P. (2002). Obstructive sleep apnoea. The Lancet, 360(9328), 237-245.

[10] Marino, M., et al. (2013). Measuring sleep: accuracy, sensitivity, and specificity of wrist actigraphy compared to polysomnography. Sleep, 36(11), 1747-1755.