The stages of the sleep cycle

To learn more about the sleep cycle, we have to know about what is going on under the hood, so to speak. So, we need some way to find out what the brain is doing, and we do this by looking at brain waves. Before you ask, you don’t need to be a psychic like Professor X or even use his telepathic boosting machine, Cerebro; you just need an EEG.

What are brain waves?

Have you ever wondered what’s happening in your brain when you sleep? Scientists have found these fascinating things called brain waves. They’re like rhythmic patterns of activity that happen all over your brain like it’s singing a song. Sometimes, it’s all upbeat and speedy when you’re awake, and other times, it’s slow and relaxed when you’re asleep. We can track these brain tunes using an EEG (electroencephalogram) headset to monitor the brain activity while you are sleeping. The best way to picture an EEG is to remember Pinhead from the Hellraiser franchise, but with the pins attached to wires. It may not be a good analogy, but it’s a good pop culture reference nonetheless. Two pop cultures references in the first two paragraphs, that’s got be some kind of record.

Your brain hums with high-frequency beta waves when you’re awake and focused. This is its normal rhythm during the day. As we transition to sleep, our brain shifts to alpha waves (high frequency and more regular peaks), signalling relaxation.

Once we become drowsy and start to fall into light sleep (non-rapid eye movement, NREM), the brain begins to sing more softly and calmly, producing a pattern of firing shown as delta waves, which are long, slow waves. The delta waves are like the slow and rhythmic sounds of the ocean caressing the beach that lulls you to sleep. Then, when we move into the stage of REM or rapid eye movement sleep, and yes, our eyes do move during this stage, the brain starts producing the same beta waves we can observe when we are awake.

How to wake up refreshed using sleep cycles

Do you want to learn how to go to sleep and wake up feeling refreshed and ready for the day? Well, read on to learn that it’s not about the number of hours you sleep but the number of sleep cycles.

In today’s sleep science, we’ve learned that a typical night’s rest involves a series of cycles, each consisting of four or five phases. We have briefly looked at the brainwaves above, but let’s delve deeper into what these stages feel like as we traverse the path to deep sleep.

Firstly, there’s the process of drifting into unconsciousness, which usually takes anywhere from five to fifteen minutes. Following that, we enter a light yet rejuvenating slumber, almost akin to taking a refreshing nap, lasting around twenty minutes. Sleep is so light during these initial stages that you might even feel awake while asleep. Next comes a deeper sleep phase, lasting roughly an hour, during which it’s much more challenging to wake someone up. Lastly, we enter the rapid eye movement (REM) phase, where most of our dreaming occurs. We pass through this cycle 4 to 6 times a night, lasting about 90 minutes. So, it’s not just a football match that lasts 90 minutes; it’s your sleep cycle, too. If you want to wake up feeling refreshed, start counting your sleep cycles rather than counting the number of hours you slept. This is because if you wake up mid-cycle, you will be groggy and want to go back to sleep, but if you wake up at the end of a cycle, you will be refreshed and ready for the day ahead. So, rather than eight hours of sleep, why don’t you try seven and a half hours, which works out to five cycles? Give it a go and see how you feel.

REM vs NREM

We have mentioned REM quite a lot, and no, it’s the devastatingly excellent alternative pop-rock group from the nineties, but it literally refers to  “Rapid Eye Movement”. For once, scientists have given a name to a phenomenon we mere mortals can understand. That’s all well and good, so we know the eyes move during a particular phase of the sleep cycle, but the why is subject to several theories.

The NREM and REM sleep types help scientists separate the different phases of sleep; again, science has made life easier because it’s pretty easy to determine which stage of sleep someone is in… look at their eyes! These terms refer to our eye movements during each stage, with the deeper REM sleep given its name due to many rapid eye movements as we dream (although we dream in NREM sleep, we are more likely to remember them if awoken during REM sleep). The two are different, and the brain cycles through them during the night, typically around 4 to 6 times each night, averaging 90 minutes for each cycle.¹

Eugene Aserinsky, the scientist who discovered REM

In December 1951, a pivotal moment in our understanding of sleep occurred at the University of Chicago. Eugene Aserinsky, a young sleep researcher, decided to test a brainwave-measuring machine his lab had obtained. Aserinsky enlisted his eight-year-old son, Armond, for his first trial as the volunteer subject.

Ninety minutes after Armond had peacefully drifted off to sleep, Eugene Aserinsky noticed something unexpected. The monitor’s graph paper suddenly came to life with jagged tracings resembling those of an alert mind. Yet, when Aserinsky checked on Armond, he was still sound asleep. However, his eyes were visibly moving beneath closed lids. At that moment, Aserinsky stumbled upon rapid eye movement (REM) sleep, the most captivating and enigmatic phase of our nightly sleep cycle.²

REM is such an interesting topic that it deserves an article of its own; we will briefly add here that it is during REM that we dream, and it has been speculated that our eyes move to follow the scenes produced in our nocturnal film production. Another important point regarding REM is that our bodies are paralysed, apart from our eyes, of course. This is a safety mechanism designed to stop us from acting out our dreams.

The five stages of the sleep cycle

Mechanism

In this next section, we will go into slightly more detail and expand on what has been mentioned previously. You will soon learn that I told a little white lie regarding the brain waves. There are a few more of them, and I didn’t mention them before because I didn’t want to scare you off. But now, since you have read this far, I guess you want to know what they are. This is the part of the article that contains the most scientific references, but I have tried to make it as painless as possible.

Sleep occurs in five stages:

• wake,
• N1,
• N2,
• N3, and
• REM.

The stages N1 through N3 are considered non-rapid eye movement (NREM) sleep; with the progression to each subsequent stage, the sleep becomes progressively deeper.

Up to 75% of sleep is spent in the NREM stages, and the majority of that time is spent in the N2 stage ³

On an average night, we experience 4 to 5 sleep cycles, with the stages unfolding in this sequence: N1, N2, N3, REM.⁴ Each complete cycle spans about 90 to 110 minutes. Initially, the first REM period is brief, but as the night advances, REM stages become longer, while time spent in deep non-REM sleep decreases.

Wake/Alert

EEG readings show beta waves, with the highest frequency and lowest amplitude, during the wake stage or stage W. This stage varies depending on whether the eyes are open or closed. When the eyes are open, beta waves prevail. Alpha waves become the dominant pattern as individuals become drowsy and close their eyes.⁵

N1 (Stage 1) – Light Sleep (5%)

Stage 1 of sleep, N1 or Light Sleep, accounts for 5% of total sleep time. EEG recordings during this stage show theta waves with low voltage. It marks the beginning of sleep when more than 50% of alpha waves are replaced with low-amplitude mixed-frequency (LAMF) activity. Muscle tone in skeletal muscles persists, and breathing remains regular. This stage typically lasts 1 to 5 minutes.

N2 (Stage 2) – Deeper Sleep (45%)

During this stage, EEG recordings show sleep spindles and K complexes, indicating deeper sleep as heart rate and body temperature decrease. Sleep spindles are short bursts of neural activity in specific brain regions, including the superior temporal gyri and anterior cingulate cortex. They play a crucial role in synaptic plasticity. Studies suggest that sleep spindles are essential for memory consolidation.⁶

K-complexes are extended delta waves lasting about one second and are among the most prolonged and distinguishable brain wave patterns. They play a vital role in sustaining sleep and aiding memory formation.⁷

N3 (Stage 3) – Deepest Non-REM Sleep (25%)

EEG recordings during stage N3 reveal delta waves, characterised by their lowest frequency and highest amplitude. N3, also termed slow-wave sleep (SWS), is considered the deepest stage of sleep, marked by these slow, high-amplitude delta waves. It’s challenging to awaken from this stage; some individuals may not respond even to loud noises (> 100 decibels). As people age, they spend less time in this deep, delta-wave sleep and more in stage N2 sleep. Despite its deepness, awakening during N3 results in a brief phase of mental fogginess known as sleep inertia, leading to moderately impaired cognitive performance for 30 minutes to an hour.⁸ This stage is crucial for bodily repair, tissue regeneration, muscle and bone growth, and immune system strengthening. It’s also associated with phenomena like sleepwalking, night terrors, and bedwetting.⁹

REM (25%)

EEG recordings show beta waves during REM sleep, similar to those observed during wakefulness. REM sleep is linked with dreaming but is not considered a deeply restful stage of sleep. While brain activity resembles wakefulness, skeletal muscles remain inactive except for the eyes and diaphragm. Breathing becomes irregular during REM sleep. This stage usually commences 90 minutes after falling asleep, with each REM cycle lengthening throughout the night. The initial cycle lasts about 10 minutes, extending to up to an hour by the final cycle.¹⁰ REM sleep is when dreaming, nightmares, and penile/clitoral tumescence occur.

Essential characteristics of REM

• It is associated with dreaming and erratic muscle movements, including rapid eye movements.
• People often wake up naturally in the morning during REM sleep episodes.
• Reduced muscle tone, elevated brain oxygen consumption, fluctuating pulse and blood pressure
• Elevated levels of acetylcholine (ACh)
• REM sleep is marked by heightened brain activity, boosting brain metabolism by up to 20%.¹¹

Summary of sleep stages

So, there you have it; you are now an expert on the stages of sleep. Understanding the sleep cycle is crucial for comprehending the complexities of sleep and its impact on overall health and well-being. By recognising the different stages of sleep, from the light slumber of Stage 1 to the deep restorative sleep of Stage 3 and the dream-rich REM stage, individuals can appreciate the intricate processes that occur during the night.

This understanding allows for insights into various aspects of health, including memory consolidation, physical restoration, emotional regulation, and cognitive function. Moreover, knowledge of the sleep cycle aids in identifying and addressing sleep disorders, such as insomnia, sleep apnoea, and restless leg syndrome, thereby promoting better sleep hygiene and quality of life.

Ultimately, grasping the nuances of the sleep cycle empowers individuals to prioritise sleep as an essential component of their overall health and adopt strategies to optimise their sleep habits for improved vitality and wellness.

References:

1. Memar P, Faradji F. A Novel Multi-Class EEG-Based Sleep Stage Classification System. IEEE Trans Neural Syst Rehabil Eng. 2018 Jan;26(1):84-95.
2. Aserinsky, E. and Kleitman, N., 1953. Regularly occurring periods of eye motility and concomitant phenomena during sleep. Science, 118(3062), pp.273-274.
3. Malik J, Lo YL, Wu HT. Sleep-wake classification via quantifying heart rate variability by convolutional neural network. Physiol Meas. 2018 Aug 20;39(8):085004
4. Feinberg I, Floyd TC. Systematic trends across the night in human sleep cycles. Psychophysiology. 1979 May;16(3):283-91.
5. Varga B, Gergely A, Galambos Á, Kis A. Heart Rate and Heart Rate Variability during Sleep in Family Dogs (Canis familiaris). Moderate Effect of Pre-Sleep Emotions. Animals (Basel). 2018 Jul 02;8(7)
6. Antony JW, Schönauer M, Staresina BP, Cairney SA. Sleep Spindles and Memory Reprocessing. Trends Neurosci. 2019 Jan;42(1):1-3.
7. Gandhi MH, Emmady PD. StatPearls [Internet]. StatPearls Publishing; Treasure Island (FL): May 1, 2023. Physiology, K Complex.
8. Hilditch CJ, McHill AW. Sleep inertia: current insights. Nat Sci Sleep. 2019;11:155-165.
9. El Shakankiry HM. Sleep physiology and sleep disorders in childhood. Nat Sci Sleep. 2011;3:101-14.
10. Della Monica C, Johnsen S, Atzori G, Groeger JA, Dijk DJ. Rapid Eye Movement Sleep, Sleep Continuity and Slow Wave Sleep as Predictors of Cognition, Mood, and Subjective Sleep Quality in Healthy Men and Women Aged 20-84 Years. Front Psychiatry. 2018;9:255.
11. Peever J, Fuller PM. The Biology of REM Sleep. Curr Biol. 2017 Nov 20;27(22):R1237-R1248.