What is Sleep Cycle? Different stages of sleep

Sleep is one of the basic human requirements and keeps us healthy. When we sleep, we think that only our body is at rest. But the truth is both our body and brain will be at rest.

Not getting enough sleep can make a person tired and will reduce cognitive function. Research states that in tired people, their parts of the brain will be asleep while the person is wide awake.

This is why we yawn and make those small errors at work while tired. Getting adequate sleep will help you recover from the previous day’s mental and physical strain. It also restores your lost energy and wakes you up fresh the next day.

What is a sleep cycle?

A sleep cycle is the stages of sleep an individual goes through. The sleep cycle goes on until the last stage, the deep sleep cycle. It is a recurring pattern of sleep in a period during which an individual will go through various slow-wave sleep called as sleep cycles.

An average sleep cycle lasts for about 90 minutes. A person will require about 6 sleep cycles. However, the sleep cycle can vary from one person to another.

To attain deep sleep, it is important to go through all stages of sleep. Sleep will benefit both physical and mental health.  

The body releases certain growth hormones during sleep, which helps repair bones, muscles and tissues. A deep sleep cycle also promotes a healthy immune system.

Stages of a sleep cycle

The sleep cycle stages include four, and the first and second stages are non-rapid eye movement sleep (NREM). The fourth and final stage of the sleep cycle is rapid eye movement (REM).

Rapid eye movement and non-rapid eye movement sleep cycles are differentiated by erratic eye movements. However, your eyelids tend to be closed during all the sleep cycle stages. There are other key categories to differentiate between the two sleep cycle stages.

NREM sleep patterns

When you tend to work on a task continuously for long hours, you eventually become tired. This is because our neurons become tired. And we tend to close our eyelids without us knowing.  

 Most NREM sleep patterns are a stage between half-wakefulness and sleepiness. Your eyes tend to close involuntarily, and you fall asleep.

The brain produces alpha waves that make you feel relaxed, and the beta waves promote a feeling of arousal.

Stage 1

Stage 1 of Non-REM sleep marks the transition between sleep and wakefulness. The alpha waves give way to a low-amplitude mixed frequency of brain activity. During stage 1 of sleep, the heartbeat decreases, and you reach a relaxed state.

Your eye movement becomes slow, and your breathing rate also goes down. At this stage, your muscles are fully relaxed, and some people experience muscle twitches.

Stage 1 of the sleep cycle is the shortest and lasts only about 5 minutes during every cycle, representing about 5 per cent of your overall sleep. NREM 1 is the first stage of sleep, which means your sleep can be easily disrupted.

Stage 2

Non-rapid eye movement 2 is the second stage of the sleep cycle. Technically, it is also considered a light-stage sleep. NREM 2 prepares the body to enter into the deep sleep stage.

In this stage, the body temperature drops, and no eye movement will occur. Breathing rate, heart rate and muscle activity will start to decrease gradually. Among all the sleep stages, it is the longest and represents about 50 per cent of the total sleep time.

The NREM 2 sleep lasts for about 25 minutes. However, during subsequent cycles, the duration decreases. During stage 2, the brain may display a burst of activity called as sleep spindles. These sleep spindles are brief and only last for about three seconds.

Sleep spindles help you avoid external stimuli and progress into the deep sleep cycle. K-complex occurs during stage 2 of the sleep cycle, which are singular delta wave bursts. This delta wave bursts last for about a few seconds.

When the transition to a deep sleep cycle occurs, the brain activity will only consist of delta waves. Some people enjoy taking naps during the day. This is because most of the snooze ends during NREM1 and NREM 2.

Waking up during stage 2 of sleep is mostly associated with alertness. If you try to wake up during NREM 3 or deep sleep, it can cause a weak, unsteady and sluggish feeling.

Stage 3

NREM 3 stage is the beginning of deep sleep. At this stage, the heartbeat and breathing rate drop to the lowest of the sleep cycle. The muscles become completely relaxed, and stage 3 of sleep is longer during the first half of the night.

Stage 3 of the sleep cycle is also called as slow-wave sleep, as the brain activity will have a slow frequency with delta waves. When we enter stage 3 of sleep, it will be difficult to awaken, and we may also sleep through noises of 100 decibels.

When we wake up at this stage, it can lead to a condition called sleep inertia. It causes mental fogginess and difficulty concentrating.

NREM 3 is divided into two separate stages, N3 and N4. The American Academy of Sleep Medicine changed and clubbed it as a single stage.

REM sleep pattern

REM stage is Rapid eye movement and differs from the other three stages. The body will be asleep. However, brain electroencephalogram (EEG) readings can indicate the brain activity of an awake person.

Blood pressure and heart rate also increase, and the eyes move side by side behind the eyelids.

During the REM stage of the sleep cycle, you tend to dream. The limbs and the body tend to undergo temporary paralysis and will restrict from any physical activity due to your dreams.

The REM stage of sleep is associated with problem-solving and memory consolidation. During the first cycle, the REM stage lasts for about ten minutes. However, as the cycle progresses, the REM stage lasts up to an hour.

What affects sleep stages ?

The sleep cycle will be affected by external factors, which alter the chemical signals in the brain. It will shift the chemical balance of neurotransmitters and prevent us from getting into the NREM 1 sleep stage.

Some of the external factors that affect the quality of sleep are

Alcohol

Alcohol will make you fall asleep but cannot reach the deep sleep stage. It also reduces the NREM and REM stages of the sleep cycle.

Caffeine and pseudoephedrine

Caffeine and pseudoephedrine are stimulants that increase your brain activity. Taking these ingredients can cause insomnia.

Medication

Medication like antidepressants can lead to reduced time in REM sleep. As a result, you will not be able to sleep properly. Your sleep cycle will be disrupted, and you will always feel sleepy. 

Hot and cold temperatures

If the room temperature is too cold or hot, it can disrupt REM sleep, and your body will spend more energy to adapt to the room temperature. As a result, your sleep will be disrupted.

How to have a healthier sleep cycle?

A healthy sleep cycle will make a person wake up fresh in the morning. Maintaining healthy sleep hygiene is the first step to a healthy sleep cycle. Sleep hygiene is a collective term for behaviour and habits that will impact sleep. To have a healthier sleep cycle, the following are required.

• Stick to sleeping time, even during weekends.
• Work out or exercise for at least 30 minutes a day. However, do not do intense workouts late evenings or close to bedtime.
• Refrain from caffeine, alcohol and nicotine from the afternoon.
• If you wake up in the middle of night, do not force yourself to sleep. Just relax and listen to gentle or soft music or read a book.
• Make your sleeping room calm and quiet with proper lighting. It is advised to sleep in a dark room.

What is a sleep debt?

Sleep debt occurs when a person does not sleep for long days. This will create a debt, and you will feel mentally and physically exhausted when the sleep debt develops. So it is important to get enough sleep every day.

What are sleep disorders?

Sleep disorders are common and will affect the quality of life. Sleep deprivation can lead to problems in work and social life.

Some of the common sleep disorders are

• Narcolepsy
• Insomnia disorders
• Sleep apnea
• Restless legs syndrome
• Snoring

Conclusion

Sleep is vital for good health. It is a recharge for the body that prepares you for the next day’s activities. There are four sleep cycle stages, and each individual’s sleep cycle will differ.

Infants require 16 hours of sleep, preschoolers and toddlers require 12 hours, and adults will require closer to 10 hours to sleep. NCBI states that an individual will go through 4 to 6 sleep cycles every night. However, the cycle will differ.

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