Rem Sleep: Stage 4 Sleep Explained

Sleep is a complex and mysterious process that is essential for our health and well-being. During sleep, our bodies cycle through different stages, including non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. These stages have distinct characteristics and play specific roles in maintaining cognitive performance and physical health. One question that arises is whether we reach stage 4 sleep within the REM cycle. To understand this, let's delve into the stages of sleep and how they unfold.

Firstly, it's important to clarify that the traditional classification of four stages of NREM sleep has been updated. Currently, NREM sleep is divided into three stages: N1, N2, and N3. NREM stage 3 encompasses what was previously known as stages 3 and 4. So, in the context of the updated classification, we do not reach a stage 4 during sleep.

Now, let's explore the sleep stages in more detail. NREM stage 1 is the initial stage when we first fall asleep. It is a light sleep stage that usually lasts a few minutes, and it is easy to wake someone during this period. NREM stage 2 is a slightly deeper sleep, where body temperature drops, eye movements stop, and breathing and heart rate become more regular. NREM stage 3, also known as deep sleep, is the hardest to be awakened from. During this stage, the body repairs and regenerates, and the immune system is strengthened.

Following the NREM stages, we enter REM sleep, which is characterised by rapid eye movements and increased brain activity. This is the stage when most dreams occur, and it is important for memory consolidation and emotional processing. The sleep cycle then repeats, starting again from NREM stage 1 or 2.

In summary, while we do not reach a designated stage 4 during sleep according to the updated classification, we progress through distinct stages of NREM and REM sleep, each serving vital functions in maintaining our physical and mental health.

The four stages of sleep

Sleep is divided into four distinct stages, which form part of the sleep cycle. This cycle is composed of three non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage.

The first stage of the sleep cycle is a transition period between wakefulness and sleep. During this stage, the brain slows down, and eye movements, breathing, and heart rate all decrease. This stage lasts for around five to ten minutes.

The second stage is a period of light sleep, where the body continues to prepare for deep sleep. The heart rate and breathing slow further, and body temperature drops. This stage can last from 10 to 25 minutes.

The third stage is deep sleep, during which it is difficult to wake the sleeper. The body relaxes completely, blood pressure drops, and breathing slows. This is the stage where the body starts to physically repair itself, and the brain consolidates declarative memories.

The fourth and final stage is REM sleep, which usually begins about 90 minutes after falling asleep. This is the primary dreaming stage of sleep, where brain activity is high, and the eyes move rapidly. The heart rate and breathing increase, and the body becomes temporarily paralysed.

After the REM stage, the cycle repeats, usually starting with NREM stage 2. A full sleep cycle is generally around 90 minutes long, and a typical night of sleep consists of four to six of these cycles.

NREM sleep

Non-rapid eye movement (NREM) sleep is an essential part of the sleep cycle, and it involves three stages: N1, N2, and N3, with N3 being the deepest. NREM sleep stages are vital for physical and mental restoration. Sleep deprivation and fragmented sleep can limit the amount of time spent in NREM sleep and lead to health problems.

Stage 1 NREM Sleep

Stage 1 NREM sleep is the first sleep stage a person enters when falling asleep. This stage usually lasts only a few minutes, making up about 5% of sleep time. During this stage, heartbeat, eye movements, brain waves, and breathing activity begin to slow down, and motor movements diminish, although muscle twitches called hypnic jerks may occur.

Stage 2 NREM Sleep

Stage 2 NREM sleep is still light sleep but deeper than stage 1. The sleeper's heartbeat, breathing, muscle activity, and eye movements continue to slow, and body temperature drops. Brain waves slow further and exhibit sleep spindles, which are essential for memory and learning, and K-complexes, which may play a role in maintaining sleep and memory consolidation. People spend about half of their total sleep time during NREM stage 2, which lasts about 20 minutes per cycle.

Stage 3 NREM Sleep

Stage 3 NREM sleep is the deepest stage of NREM sleep. It makes up about 25% of total sleep time in adults, but the percentage is higher in babies and children, and it decreases with age. During this stage, the sleeper's muscles are completely relaxed, blood pressure drops, and breathing slows. The brain produces slow, strong brain waves known as delta waves, and the body starts its physical repairs. Declarative memories are also consolidated during this stage.

REM sleep

The sleep cycle consists of four stages, the first three being non-REM (NREM) sleep. NREM sleep is further divided into three substages: N1, N2, and N3. The first stage, N1, is a transition period between wakefulness and sleep, lasting only a few minutes. During this stage, the body begins to slow down, with a decrease in eye movement, muscle activity, heart rate, and breathing. N1 is followed by N2, a deeper stage of sleep where body temperature drops and eye movements stop. The final stage of NREM sleep is N3, also known as deep sleep, which is harder to be awakened from. In this stage, the body performs essential functions such as tissue repair, growth, and strengthening the immune system.

After progressing through the three stages of NREM sleep, the body enters REM sleep. This stage typically occurs about 90 minutes after falling asleep and lasts for approximately 10 minutes. Each subsequent REM stage gets longer, with the final one lasting up to an hour. During REM sleep, the body is temporarily paralysed, except for the eyes and breathing muscles. The heart rate and blood pressure increase, and the eyes move rapidly.

Sleep disorders

• Sleep apnea: A breathing disorder characterised by pauses in breathing during sleep, lasting 10 seconds or more.
• Restless leg syndrome: A tingling or prickly sensation in the legs, accompanied by a powerful urge to move them.
• Hypersomnia: The inability to stay awake during the day, including narcolepsy, which causes extreme sleepiness.
• Circadian rhythm disorders: Problems with the sleep-wake cycle, resulting in difficulty sleeping and waking at the right times.
• Parasomnia: Unusual behaviours during sleep, such as walking, talking, or eating.

Improving sleep quality

• Stick to a sleep schedule: Try to go to bed and wake up at the same time every day, even on weekends. This helps to regulate your body's internal clock and improve your sleep quality.
• Create a relaxing bedtime routine: Wind down with a relaxing activity before bed, such as reading, listening to soothing music, or practising relaxation techniques like deep breathing or meditation.
• Limit exposure to blue light before bed: Blue light from electronic devices can interfere with your sleep. Try to avoid screens for at least an hour before bedtime, or use blue light filters or night mode on your devices.
• Get natural light during the day: Exposure to natural light during the day can help regulate your body's sleep-wake cycle. Try to get outside or sit by a window during the day, especially in the morning.
• Exercise regularly: Daily exercise can improve your sleep, but avoid intense exercise close to bedtime as it may hinder your ability to relax and fall asleep.
• Avoid caffeine and nicotine: Caffeine and nicotine are stimulants that can disrupt your sleep. Limit your intake, especially in the afternoon and evening.
• Avoid alcohol: Alcohol may induce drowsiness, but it can also lower sleep quality and disrupt your sleep patterns. Avoid drinking alcohol close to bedtime.
• Optimise your bedroom environment: Make sure your bedroom is cool, dark, and quiet. Consider using blackout curtains, earplugs, or a white noise machine to create a comfortable sleep environment.
• Invest in a comfortable mattress and bedding: A supportive mattress, pillow, and bedding can help you get a good night's sleep. Look for bedding that feels comfortable and maintains a cool temperature.
• Limit naps: Long or irregular naps during the day can disrupt your nighttime sleep. If you must nap, try to limit it to 20 minutes in the early afternoon.
• Avoid heavy meals and liquids before bed: Eating or drinking too close to bedtime can disrupt your sleep. Try to finish dinner a few hours before bed and avoid liquids at least one to two hours before sleeping.
• Try supplements: Melatonin and other natural supplements like valerian, chamomile, and glycine can help you relax and improve your sleep. Always consult a healthcare professional before taking any supplements.

Frequently asked questions