Science Behind Sleep: Exploring the stages of sleep and their benefits

Sleep plays a crucial role in our overall well-being. It is a key part of our daily routine, but have you ever wondered what happens when we close our eyes? In this article, we'll break down the different stages of sleep, what they mean for our health and things that might affect them, as well as the benefits each stage provides us. 

What are the five stages of sleep?

Simply put, there are five stages of sleep that we will experience during the night (Patel et al, 2022). Here is a brief rundown of each stage of sleep:

Stage 1 (N1)

This is the lightest stage of sleep, a transition phase where you move from wakefulness to sleep. It's a short period, and during this time, your heartbeat, breathing, and eye movements slow down.

Stage 2 (N2)

This is still a light form of sleep, but you're gradually drifting into a more restful state. Heartbeat and breathing continue to slow, and body temperature drops. Eye movements stop, and you're less likely to be awoken by external disturbances.

Stage 3 (N3)

Commonly referred to as deep sleep or slow-wave sleep. It's harder to be awakened from this stage, and it's when the body repairs muscles, grows tissue and boosts immune function.

Stage 4 (N3 Continued/Back to N2)

This is a continuation of the deep sleep stage. It plays a crucial role in making you feel refreshed in the morning. As with the earlier part of N3, it's a time for physical recovery and growth. This stage may also revert back to N2 sleep.

REM (Rapid Eye Movement) Sleep 

This is when most dreaming occurs. The brain becomes more active, while muscles become very relaxed, almost to the point of paralysis. Heart rate and breathing become irregular. REM sleep plays a key role in brain function and memory.

Video: How does sleep work and why do we need it?

Sleep Is The Foundation have provided a useful video to explain the role that each stage of sleep plays in allowing your body and mind to feel refreshed when you wake up. You can watch it below:

Which stages of sleep are considered light sleep?

Light sleep encompasses two specific stages, Stage 1 (N1) and Stage 2 (N2). Stage 1 (N1) is the lightest stage of sleep and begins when more than 50% of the alpha waves are replaced with low-amplitude mixed-frequency (LAMF) activity (Patel et al, 2022). Stage 2 (N2) is a period of light sleep that occurs before you enter deeper sleep.  Your heartbeat and breathing slow down, and your muscles relax even further.  Your body temperature drops and eye movements stop.  Brain wave activity slows but is marked by brief bursts of electrical activity. 

Which stages of sleep are considered deep sleep?

Stages 3 and 4 are known as deep sleep or slow-wave sleep stages. According to the National Heart, Blood and Lung Institute, you usually spend more time in these stages earlier in the night. In both these stages, it's more difficult to wake someone up, and if awakened, a person might feel groggy or disoriented for a few minutes. These stages are crucial for physical recovery, tissue repair, and immune system function.

Which stage of sleep does vivid dreaming take place?

Vivid dreaming predominantly takes place during the REM (Rapid Eye Movement) stage of sleep. Vivid dreaming predominantly takes place during the REM (Rapid Eye Movement) stage of sleep. During REM sleep, the brain is highly active, resembling the brain's activity when awake. This stage is characterised by rapid movement of the eyes, increased heart rate, and irregular breathing. Along with vivid dreams, muscle paralysis occurs in this stage, which is believed to be a protective mechanism to prevent us from acting out our dreams physically.

What happens in N1 sleep (stage 1)?

N1 Sleep, or Stage 1, is the initial phase of our sleep cycle, acting as a gentle bridge between wakefulness and deeper sleep stages. Lasting only a few minutes, it's characterised by a light sleep from which individuals can be easily awakened. In this stage, our heart rate begins its descent, and our muscles start to relax, sometimes accompanied by sporadic twitches. Slow, rolling eye movements are typical, and the brain shifts from the alert alpha waves of wakefulness to the more relaxed theta waves. If someone is roused during this stage, they might not even feel as though they've been asleep.

What happens in N2 sleep (stage 2)?

N2 Sleep, or Stage 2, marks a deeper level of sleep. This stage often accounts for about half of an adult's total sleep duration. Within N2, our heart rate and body temperature continue their downward trend, signalling increased relaxation. The characteristic eye movements of the previous stage fade away. Distinctly in this phase, brain activity showcases sleep spindles and K-complexes, both of which play roles in memory consolidation and shielding sleep from external disruptions. 

What happens in N3 sleep (stage 3)?

N3, commonly known as Stage 3, lays the groundwork for bodily rejuvenation. In this sleep state, both the heartbeat and respiration rate are notably subdued, and our muscles achieve optimal relaxation. The brain's dominant activity is marked by delta waves, which are indicative of the deepest levels of sleep. Disturbing someone in this phase might result in them feeling dazed or muddled. 

What happens in REM sleep?

REM Sleep, also known as Rapid Eye Movement sleep, is the dream centre of our nightly rest. In this phase, even though the brain is lively and bustling, much like its daytime activity, our body takes a break. Our eyes, true to the stage's name, dart about rapidly, yet our major muscles are in a state of near paralysis. This is the mind's prime time for processing daily experiences, emotions, and memories. 

Are all sleep cycles the same?

There are two basic types of sleep:  rapid eye movement (REM) sleep and non-REM sleep (which has three different stages). We will all experience these sleep cycles most nights when we sleep. This does not mean that all sleep cycles are the same. While each cycle contains the stages of sleep (N1, N2, N3, and REM), the duration and intensity of each stage can vary across the cycles during a night.

What factors can affect the sleep stages?

Age

As individuals age, their sleep patterns undergo notable changes. Older adults tend to sleep and wake earlier, experience shorter night-time sleep with increased awakenings, and have reduced deep (slow-wave) sleep. While many of these shifts occur predominantly from young to middle adulthood, the overall sleep patterns in healthy elderly individuals remain fairly stable (Li et al, 2017). 

Recent sleep patterns

Changes in sleep schedule and in the quantity and quality of night-time sleep can affect your sleep cycles (Gupta et al, 2020). Any significant change in sleep patterns can affect the structure, duration, and quality of sleep cycles, which in turn influences overall health and well-being.

Alcohol consumption

Many people use alcohol as it can have sleep-promoting effects. Nonetheless, alcohol disrupts sleep through multiple mechanisms, such as disrupting electrophysiologic sleep architecture, triggering insomnia, and contributing to abnormalities of circadian rhythm (He et al, 2019).

Sleep disorders

Sleep disorders can significantly disrupt regular sleep cycles. Conditions like insomnia may result in difficulties falling asleep or staying asleep, shortening the overall duration of rest. Sleep disorders are common in both adults and children. However, children with sleep disorders may present with different symptoms than adults (Karna et al, 2023).

Sources

• Stages of Sleep - National Library of Medicine (UK)

• The Four Stages of Sleep - Verywell Health

• Stages of Sleep - Healthline