Last updated: August 15, 2026 · 8 min read
What is Sleep?
We spend a significant portion of our days and our lives sleeping. It might be the activity that we spend the most time on. But what is it? It is one of those aspects of life that we are so used to that we give little thought to. However, sleep is mentioned as one of the core pillars of a healthy lifestyle alongside other elements such as proper nutrition and exercise.
Sleep quality and duration have impacts on work stress, general satisfaction with life, and subjective accounts of health and well-being. The importance of sleep is perhaps best appreciated when its absence is felt. There are significant adverse effects on physical, mental, and overall well-being when one is sleep-deprived. Sleep deprivation is affected by the time slept and the quality of sleep. It plays a significant role in determining various aspects of personal well-being.
While much is known about sleep's effects and its extensive role in determining health, defining what sleep is and why it exists remains a puzzle.
In This Post
- Definition
- Sleep Stages
- Why Do We Sleep?
- Sleep Disorders
- Improving Sleep Hygiene
- Frequently Asked Questions
Definition
Sleep may be defined as a naturally occurring state of rest. It involves altered brain activity, reduced or practical suspension of consciousness of the surrounding environment, and relaxation of muscles involved in posture. In this state, the brain does not react to most external stimuli. Closing the eyes during sleep helps establish this state, which may explain how important visual stimuli are for wakefulness. Furthermore, the natural sleep cycle involves sleeping at night. Visual stimuli are limited during this period since there is little or no light.
This may raise some additional questions: why did we evolve to sleep? Sleep must be necessary, given the time we spend doing it. Therefore, its risks, such as the likelihood of being devoured by a predator while asleep at night, must be lower than the benefits of sleep. Also, how common is sleep among mammals or in the animal kingdom? Sleep is ubiquitous in the animal kingdom in almost all but the simplest organisms without a brain or those with a rudimentary brain. Given the variety of ecosystems in which animals that sleep live, one may suggest that sleep does not require a narrow set of conditions. From penguins in Antarctica to the guanacos in the Atacama, nature has bestowed these organisms with the ability to enjoy the benefits of sleep regardless of their environment.
Sleep Stages
While it may be tempting to think of sleep as one uniform state that one falls into, we have established that sleep has several phases. The sleep cycle has four stages. The first stage is short. It is non-REM — it does not involve rapid eye movement (REM). In this stage, the brain activities slow down, and it is easy to wake someone up in this stage. It often lasts about seven minutes. It is followed by another stage that is also non-REM. In this stage, which lasts up to 25 minutes, the body becomes more relaxed. The body temperature drops and the muscles relax. The breathing rate and heart rate slow. The third stage involves deep sleep. The body relaxes further and may last up to 40 minutes. It is also non-REM. The fourth stage is REM sleep. It lasts up to an hour. As you can see, the sum of all the times in each stage is less than the eight hours we should sleep in a twenty-four-hour period — one cycles through these stages throughout the night.
During REM sleep, there is significant brain activity alongside temporary paralysis. Most muscles are paralyzed except the eyes and those that control breathing. Even when the eyes are closed, there is considerable eye movement. As a result, this stage gets its name — rapid eye movement. But why do eyes need to move even when shut during this stage? This stage is believed to be essential for brain functions such as memory, cognitive, and learning functions. The dreams in this stage are more common and intense than in non-REM stages.
Why Do We Sleep?
One of the physiological benefits of sleep is allowing for physical restoration. During sleep, energy and resources are directed towards functions such as tissue repair and fostering immunity. During sleep, the body establishes a pattern of healthy hormonal changes, resulting in a healthy balance. Sleep also affects how the body reacts to insulin. Lack of sleep can, therefore, result in higher blood sugar levels and predispose someone to diabetes. Sleep also triggers the release of growth hormones. This is especially crucial in children with much faster growth rates than adults — I do not know if you believed your height grew by a few millimeters when asleep because you could not observe growth during wakefulness. Sleep also helps the brain improve its functions. It develops new pathways to help in learning, memory, and problem-solving.
The brain plays a vital role in determining the cycle of sleep. The hypothalamus controls sleep by processing sensory information about light. As light decreases in the environment, the suprachiasmatic nucleus signals the pineal gland to increase the hormone melatonin, increasing sleep likelihood. During sleep, the thalamus slows down, allowing one to tune out from the world and sleep. However, during REM sleep, activity in the thalamus picks up. The amygdala, which is involved in processing emotions, is also active during REM sleep. The brainstem plays a role in relaxing muscles to enable sleep and prevent that fabled sleepwalking, or worse, working.
Given the importance of sleep for physical and mental health, it is essential to have enough sleep. However, we often fail to achieve the desired level of sleep in quantity and quality. The results can be mild or catastrophic. They range from dull anxiety and mood swings to falling asleep when driving or engaged in other critical activities where the loss of consciousness can be disastrous. It results in trouble learning, focusing, and participating in social relationships. It also exacerbates the likelihood of chronic illnesses such as diabetes, high blood pressure, stroke, obesity, and kidney and heart diseases.
Sleep Disorders
One of the most common sleep disorders is sleep apnea. It arises from the willingness but inability to fall asleep. People with insomnia cannot stay awake during the day or when they are supposed to be active. Sleep apnea is associated with snoring and disruptions to breathing patterns, resulting in a blocked airway. It may result in poor sleep quality. Narcolepsy is a sleep disorder characterized by excessive tiredness that makes one have an overwhelming urge to sleep during the day. This is associated with a disruption in the sleep-wake cycle. It results in sleep attacks that often last for several minutes. Restless legs syndrome occurs due to crawling or tingling sensations in the legs, resulting in movements that make it difficult to sleep. It may be linked to iron deficiency, Parkinson's disease, and pregnancy. Parasomnias refer to unusual sleep behaviors before, during, or when waking up. They include bedwetting, night terrors, and sleepwalking.
Improving Sleep Hygiene
Sticking to a sleep-wake schedule is one of the best ways to ensure quality sleep every night. Stick to the time you go to bed and wake up. Over time, the internal clock will become so established that you will not need an alarm to wake you up. It is also essential to time your meals well so you are not hungry or stuffed as you go to bed. This may result in discomfort that keeps you up.
Also, caffeine, nicotine, and alcohol consumption can disrupt the sleep cycle. Further, the sleep environment should be optimized for sleep. It should be cool, quiet, and dark. Physical activity also helps promote better sleep. Stress management practices such as meditation and journaling can help minimize worries close to bedtime, resulting in better quality sleep. If sleep hygiene alone isn't cutting it, it's worth exploring whether a specific issue — like insomnia or trouble falling asleep despite exhaustion — is at play.
A Note From the Start of 8in24
This was the very first post on 8in24 — the questions it raises (what sleep actually is, why we do it, and how to protect it) are the same ones this whole blog has grown around since. If you're new here, the Complete Guide to Insomnia and Sleep Disorders Overview hub pages are good next stops.
Frequently Asked Questions
What are the four stages of sleep?
Sleep moves through two light non-REM stages, a deep non-REM stage, and REM sleep, cycling through all four roughly every 90 minutes throughout the night.
Why do we need sleep at all?
Sleep supports physical restoration like tissue repair and immune function, regulates hormones including those affecting blood sugar and growth, and helps the brain consolidate memory and learning.
What's the difference between insomnia and sleep apnea?
Insomnia is difficulty falling or staying asleep despite the opportunity to do so, while sleep apnea involves physical disruptions to breathing during sleep, often accompanied by snoring, that fragment sleep even when the person doesn't fully wake.
What's the single most effective sleep hygiene habit?
Sticking to a consistent sleep-wake schedule is generally considered the most foundational habit, since it trains the body's internal clock and makes falling asleep and waking up progressively easier over time.