Sleep Stages and Sleep Cycles: Mechanisms and How They Work

Sleep plays a central role in your quality of life, because it influences your physical and mental health and your energy levels day to day. Sleeping too little or having “bad nights” affects what is known as sleep health. Sleep is a natural physiological process, essential for good intellectual functioning, productivity, the immune system and the processes by which cells rebalance and regenerate.

Once you understand the mechanisms of sleep, such as the circadian rhythm and sleep pressure, it becomes easier to take steps to sleep better. Many of the body’s functions are cyclical and sleep is no exception: it is organised into sleep cycles, and each cycle is made up of different sleep stages.

There are two main types of sleep: non-REM sleep, which includes the stages of light sleep and deep sleep, and REM sleep, the stage associated with dreaming. In this article we explain how they work, how long they last and what you can do to protect your rest.

What is sleep and what is it for?

Sleep is a natural physiological state that is essential to life. It is a key component of physical and mental health, and prolonged sleep deprivation has serious consequences for the body.

The biological processes of sleep are regulated by sleep pressure and the circadian rhythm. These are the mechanisms that make us feel sleepy, fall asleep, stay asleep and, finally, wake up. Neurotransmitters are the chemical messengers that act as signals to activate the rhythms and stages of sleep and wakefulness.

The functions of sleep are many. During sleep, the body carries out important restorative functions, such as restoring energy and regulating metabolism, and sleep also has a regenerative role and helps consolidate memory, according to SNS 24.

Getting enough sleep at a suitable time contributes to:

• the proper functioning of the immune system;
• hormonal and metabolic regulation;
• cardiovascular balance;
• brain function, including memory, learning, concentration and productivity;
• mood and emotional wellbeing.

The release of growth hormone is linked to sleep: the most consistent peak occurs shortly after falling asleep, during the first period of deep sleep, a relationship present from early childhood into adulthood. In children and teenagers, insufficient or poor-quality sleep undermines healthy development and health, SNS 24 warns.

The mechanisms of sleep: circadian rhythm and sleep pressure

Many of the body’s functions follow regular rhythms. Nearly every tissue and organ has its own circadian rhythm, synchronised by a master clock in the brain.

Our day alternates between wakefulness and sleep. We are governed by the circadian cycle or circadian rhythm, the physical, mental and behavioural changes that follow a cycle of roughly 24 hours.

The circadian rhythm, which tells us “when” to sleep, follows the cycle of light and darkness, and exposure to light is, according to the NINDS, its greatest influence. Throughout the day and night, the brain produces neurotransmitters and hormones that regulate falling asleep, staying asleep and waking up.

Sleep pressure, also called sleep homeostasis, is the need for sleep that builds up during the hours we are awake. According to the NINDS, this drive gets stronger every hour we stay awake and makes us sleep longer and more deeply after a period without sleep.

Melatonin, serotonin and GABA: the messengers of sleep

The light captured by the eyes is relayed to the brain’s master clock, the suprachiasmatic nucleus, which controls the production of melatonin according to the amount of light received. Melatonin is a hormone released by the pineal gland that helps you feel sleepy when it gets dark.

With morning light, melatonin production falls. Serotonin is one of the neurotransmitters involved in regulating sleep and wakefulness, while GABA, produced by sleep-promoting cells in the brain, reduces activity in brain areas linked to alertness.

What are the sleep stages?

The sleep stages are distinct physiological phases the body goes through while we sleep. The two main types are REM sleep (Rapid Eye Movement) and non-REM sleep (NREM). Together, they form a sleep cycle.

Non-REM sleep: the three stages

Non-REM sleep is the type of sleep that lasts longest and is divided into three stages:

• Stage 1, transition: this is the changeover from wakefulness to sleep and usually lasts several minutes. Heartbeat, breathing and eye movements slow down and the muscles relax, sometimes with occasional twitches.
• Stage 2, light sleep: this is the stage in which we spend the most time across our sleep cycles. Heartbeat and breathing slow further, body temperature drops, eye movements stop and brain activity slows, according to the NINDS.
• Stage 3, deep sleep: this is the sleep you need to feel refreshed in the morning. Heartbeat and breathing reach their lowest levels and it is harder to wake up. According to the NHLBI, we spend more time in this stage early in the night.

REM sleep: the dreaming stage

During REM sleep, the eyes move rapidly behind closed eyelids and brain activity comes close to that of wakefulness. The first REM period occurs about 90 minutes after falling asleep, and this is the stage in which most dreaming happens. The arm and leg muscles become temporarily paralysed, which stops the body from acting out dreams.

REM sleep is associated with memory formation, although memory consolidation most likely requires both REM and non-REM sleep, according to the NINDS. As we age, we spend less time in REM sleep.

Sleep cycles: how do they work?

Each sleep cycle includes the non-REM stages followed by REM sleep and starts over every 80 to 100 minutes, according to the NHLBI. There are usually four to six cycles per night. As the night goes on, deep sleep takes up less time and REM periods become longer.

How many hours of sleep do you need at each age?

The recommended sleep duration varies with age: the younger the person, the more hours of sleep they need. According to the recommendations of the CDC and SNS 24:

• Newborns (0 to 3 months): 14 to 17 hours a day;
• Pre-school age (3 to 5 years): 10 to 13 hours a day, including naps;
• Teenagers (13 to 18 years): 8 to 10 hours a day;
• Adulthood: 7 or more hours a night on a regular basis, according to the American Academy of Sleep Medicine consensus; the NINDS states that most adults need 7 to 9 hours;
• Older age: the CDC recommends 7 to 8 hours from the age of 65. With ageing, sleep tends to be lighter, with more night-time awakenings and less time in deep sleep.

Sleep patterns refer to the way sleep is distributed over 24 hours: timing, duration and how the sleep cycles and stages are organised. They vary from person to person, depending on age and lifestyle, and are influenced by biological, behavioural and environmental factors.

Understanding how night-time cycles are organised and how they change over the course of life helps you adopt habits that promote more restorative sleep, suited to the needs of each stage.

Disorders that disrupt sleep stages and cycles

Sleep disorders are common. They disrupt the sleep cycles or the mechanisms of sleep and therefore compromise sleep health, wellbeing and quality of life. Among the best known are insomnia, sleep apnoea and narcolepsy.

Insomnia

Insomnia is a common sleep disorder. It involves difficulty falling asleep, staying asleep or getting good-quality sleep. According to the NHS, stress, anxiety and depression are among the most common causes, along with factors such as noise, caffeine, alcohol, shift work and jet lag.

Sleep apnoea

Sleep apnoea is a condition in which breathing stops and starts during sleep, and the most common type is obstructive sleep apnoea. According to SNS 24, it is characterised by brief but frequent blockage of the upper airways, with complete or partial pauses in breathing, and it causes repeated awakenings and restless, unrefreshing sleep. Being overweight, a larger neck circumference, age, smoking and drinking alcohol are among the associated factors.

Narcolepsy

Narcolepsy is a chronic neurological disorder that affects the brain’s ability to control the sleep-wake cycle. It causes intense sleepiness during the day and episodes of falling asleep suddenly. It can interfere with work and driving: according to the NHS, whether you can drive depends on your symptoms and treatment.

In general, a chronic lack of sleep or poor-quality sleep increases the risk of health problems such as high blood pressure, cardiovascular disease, diabetes, depression and obesity.

How to respect your sleep stages and sleep better

Small changes to your routine can help protect your sleep cycles and make your rest more restorative:

• Regular times: going to bed and getting up at the same time every day, as the CDC recommends, helps to stabilise the circadian rhythm.
• Natural light in the morning and little light at night: morning daylight helps your internal clock align with the day, while light from electronic devices at night can confuse the body clock. The CDC advises turning off screens at least 30 minutes before bedtime.
• Be careful with stimulants: avoid caffeine in the afternoon and evening, as well as large meals and alcohol before bed. The NHS advises avoiding coffee, tea and alcohol for at least 6 hours before going to bed.
• A dark, quiet and cool bedroom: the CDC and the NHS recommend a quiet, dark bedroom at a cool, comfortable temperature, which makes it easier to fall asleep and stay asleep.
• A comfortable bed: a mattress suited to your body and a memory foam pillow that supports your neck can make your rest more comfortable.

The air quality in your bedroom also matters. Night-time respiratory symptoms, such as the nasal congestion of allergic rhinitis or allergic asthma attacks, can disturb your sleep. According to the NHS, asthma symptoms may be worse at night and allergic rhinitis can affect sleep and everyday life.

Washing your bedding regularly at 60°C or above and dusting with a damp cloth help to reduce exposure to dust mites, according to the NHS. A Defender air purifier in the bedroom can complement these measures: according to the US EPA, portable air cleaners can filter particles from the air, although none can remove every pollutant.

When should you see a doctor?

An occasional bad night is normal. However, you should talk to your doctor if you notice any of these signs:

• difficulty falling or staying asleep that has lasted for months or that affects your daily life, even after improving your sleep habits;
• loud snoring, pauses in breathing or choking noises during sleep, often noticed by the person sleeping next to you;
• excessive daytime sleepiness or episodes of falling asleep suddenly, especially if you drive;
• persistent tiredness during the day, even after sleeping your usual number of hours.

Frequently asked questions about sleep stages

How many sleep stages are there?

There are four sleep stages: three stages of non-REM sleep (transition, light sleep and deep sleep) and one stage of REM sleep, associated with dreaming. These stages repeat in cycles throughout the night.

How long does a sleep cycle last?

Each sleep cycle lasts about 80 to 100 minutes, according to the NHLBI. Over a full night, it is usual to go through four to six cycles.

Which sleep stage is the most important?

Every stage of sleep has its own functions. Deep sleep is what allows you to wake up refreshed, while memory consolidation appears to depend on both REM and non-REM sleep. What matters most is that the cycles run their full course without frequent interruptions.

How many hours should an adult sleep?

The consensus of the American Academy of Sleep Medicine and the Sleep Research Society recommends that adults sleep 7 or more hours a night on a regular basis, and the NINDS states that most need 7 to 9 hours. Needs vary from person to person.

Sleep stages and quality of life: the essentials

Knowing the sleep stages, their mechanisms and what makes a healthy cycle makes the importance of sleep for physical, mental and emotional balance clearer. Preserving the mechanisms of sleep, namely sleep homeostasis and the circadian rhythm, contributes to the balance of the body’s systems and functions and to greater vitality.

Deep sleep and REM sleep contribute to physical recovery and memory consolidation. It is up to you to look at your habits and routines and identify the changes you can make to improve your sleep and, with it, your quality of life.

If you are looking for more comfortable rest, discover the mattresses and pillows in the Filtrarte Dreamline range, designed to support your body through every stage of sleep.

A good night’s sleep is priceless!

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By the Filtrarte Expert Team
The information in this article is not intended to recommend any treatment for symptoms or illnesses. Consult your doctor for specialist advice.