Sleep Basics: Understanding Sleep Stages, Mechanism, Importance

Sleep is important for numerous reasons, but most importantly it allows the body time to rest and repair itself. Sleep deprivation occurs when you don’t get enough sleep for one or more nights for any reason. Sleep deprivation has been associated with a host of negative effects on health, including increased risk of cardiovascular disease, diabetes, obesity, mental illness, and even death. Prolonged sleep deprivation may result in a diagnosis of insomnia.

Insomnia is a sleep disorder characterized by a sustained difficulty in falling asleep. Insomnia can be acute or prolonged and is also associated with a large number of negative health effects. Improving sleep quality is essential for those who deal with insomnia or other sleep disorders, as you need to make the most of the sleep you are able to get.

Sleep quality is a measure of how well you sleep, not just how long you sleep. People who wake up feeling refreshed after 5-6 complete sleep cycles likely have a good sleep quality. Doctors and scientists use three main tests to determine how well a person sleeps, including Electroencephalography (EEG), Electrooculogram (EOG), and the Electromyogram (EMG). These tests for evaluating each sleep quality metric are listed below.

Electroencephalography, or EEG as it is more commonly known, is a method of measuring brain activity by recording electric currents using electrodes placed on the scalp. The device that records the EEG data is called an electroencephalograph – or EEG for short.

The scoring of the data is based on a standard with 16 different frequency bands, categorized into four main groups: delta, theta, alpha, and beta. Each band has a different function and can be correlated with certain behaviors like falling asleep or waking up.

An Electrooculogram (EOG) records the difference in electrical potential between the front and back of the eye. This is known as a “look” and corresponds to an individual's pupils. Look amplitudes vary with pupil size, since their size changes in response to light intensity, mental activity or interest, fatigue, drug use, emotions, and many other internal and external factors.

An electromyogram (EMG) is a test used to measure and record the electrical activity of skeletal muscles at rest and during contraction. It tests for neuromuscular diseases as well as some forms of muscular disease. However, when used to measure sleep quality, it can indicate the extent of physical stress on the skeletal muscles during sleep and show periods of movement and restlessness.

There are four stages in a complete cycle of sleep. These include Stage 1, Stage 2, Stage 3, and REM (rapid eye movement) sleep. Each stage is characterized by a different brain wave pattern and distinct physiological changes. During each stage, the body moves through different phases of sleep including non-rapid eye movement (non-REM), rapid eye movement (REM), and transitional stages between them.

Stage one occurs as a person is falling asleep. Stage one is characterized by drowsiness and the slowing down of brain waves from their waking frequency, as well as a slowing of the heart rate to a resting rate which will remain stable until REM sleep begins. Muscles begin to relax in this phase, though some twitches may occur. Sporadic bursts of electrical activity called Sleep Spindles and K-Complexes are also present. Stage one of sleep accounts for 5 percent of total sleep. During this time it is common for a person to experience hypnagogic hallucinations, sleep paralysis, and night terrors as the brain begins shutting off signals from the spinal cord.

Stage two occurs as a person reaches the first stage of actual sleep; characterized by further slowing of brain waves and the absence of Sleep Spindles and K-Complexes. In stage two, the heart rate stays at rest, and breathing slows down even further. Stage two accounts for about 50 percent of total time spent asleep. A person in Stage 2 is difficult to wake up, even in response to loud noises or bright lights.

Stage three is known as “slow-wave sleep” (SWS) or “deep sleep,” characterized by high amplitude and low-frequency brain waves, and the lowest heart and breathing rates. Stage three is known as restorative sleep, where the body is able to repair itself so you can wake up feeling rested. About twenty percent of the total time asleep is spent in stage three. Sleep Spindles and K complexes may also appear in this phase occasionally.

Rapid eye movement, or REM, is the final stage in a complete cycle of sleep; occupying about 25 percent of total time asleep, though the REM phase gets longer with each cycle. REM sleep is characterized by irregular and chaotic brain waves similar to those experienced during the waking state. Also present are rapid eye movements – hence the name of this stage. A person may also experience a slightly elevated heart rate compared to the previous stages. A person experiencing REM is generally difficult to wake up due to the deep sleep experienced in Stages 3 just prior.

REM sleep is a period of “rapid eye movement” that occurs in the last stage of sleep. In this phase, the heart and breathing rates increase, and brain activity also increases. This phase is essential for mental performance the following day and memory consolidation. This phase is also called “paradoxical sleep” because brain activity is most intense (as seen on an EEG) during this state when the body's muscles are paralyzed. REM sleep is a unique and mysterious stage in which dreams are most vivid. It is during this stage that the brain, specifically the pons, sends signals to keep muscles paralyzed – an essential part of sleep for survival because it prevents one from acting out their dreams.

Dreams are a series of thoughts, images, and sensations occurring in a person’s mind during sleep. The relationship between REM sleep and dreaming is that dreaming occurs almost exclusively during REM sleep. In fact, it is thought that dreaming can't happen without REM sleep because the pons – which sends signals to the muscles telling them to stay paralyzed – also sends a signal to a part of the brainstem called the locus coeruleus. The noradrenaline from this section of the brainstem stimulates the cerebral cortex, in particular, layer IV, which is responsible for integrating and organizing information from all of our senses.  

Asking “why do we dream” is a little more complicated. Some experts believe that during REM, layer IV integrates our perceptions with memories and emotions to create a story we call a dream. It is widely believed that the brain stem induces REM sleep, and the forebrain creates dreams. 

The sleep mechanism works by regulating two separate yet interrelated processes: the sleep-wake cycle and the circadian rhythm. The sleep-wake cycle is controlled by a part of our brain called the Suprachiasmatic Nucleus (SCN) in conjunction with other areas that are involved in regulating mood, behavior, hunger, thirst, etc.

All together these work to produce a circadian rhythm, which is a process that regulates various biological functions such as temperature and hormone release. The circadian rhythm works on a 24-hour cycle and humans are most productive during the day and least productive at night – just like many other animals.

The quality and length of sleep are influenced by a variety of physical and mental factors. Generally speaking, mental health issues and physical health conditions greatly influence the amount of rest we get nightly, but they’re not the only factors. Your regular sleep schedule, environment, and the things you consume also have a great effect on improving sleep quality.  

1. Irregular Sleep Schedule: An irregular sleep schedule is any sleep schedule that does not occur routinely. Even if you go to sleep at the same time each night, if you are constantly changing your wake time, or take inconsistent naps during the day, you may struggle to sleep for long enough at night, and the sleep you get may not be high-quality rest. Instead, aim to sleep at the same time each night, and get up at the same time each day. Additionally, try to sleep for four to six complete sleep cycles rather than waking mid-cycle. 
2. Sleep Environment: Your sleep environment should be relaxing and comfortable, neither too hot, nor too cold, and dark enough to avoid stimulation. If you work in the same place you sleep, you may struggle to transition from active periods to periods of rest. It is better to sleep in a quiet place, with low lighting, and few distractions to keep you awake. 
3. Caffeine and Alcohol Intake: Caffeine typically has a negative effect on both the quantity and quality of sleep. This is because it’s a stimulant that can keep you up at night, but it also makes your mind more active when you do fall asleep. Using caffeine as an energy boost isn’t always bad (for example, many people drink coffee to stay alert in the morning), but it can be habit-forming if you’re constantly using it as a crutch.  Likewise, alcohol has a sedative effect on the body that can make you fall asleep much easier. If consumed before bed, however, this causes disrupted sleep through frequent bathroom trips and irregular sleep cycles.
4. Diet: Foods that may disrupt sleep cycles and keep you up at night include spicy foods, foods high in fat, and sugary foods that can cause blood sugar peaks and drops. Digestion slows down when we sleep, and if we go to bed too full or too hungry, we’re likely to wake up disturbed due to either hunger pains or indigestion depending on what we eat right before bed. 
5. Mental Health State: Being stressed or anxious about something can greatly affect your mental state and your sleeping patterns. It’s been scientifically documented that people with mood disorders exhibit disrupted sleep at night, but even those without a diagnosed mental illness can also experience this if they’re consumed by stress or anxiety.
6. Sleep Disorders: Those with sleep disorders such as sleep apnea, narcolepsy, and restless leg syndrome will always experience shortened or interrupted sleep without treatment. With treatment, however, many people should be able to attain a healthy sleep quality and quantity. 
7. Other Health Conditions: Underlying medical conditions will also influence how well a person sleeps, depending on the symptoms presented. For example, those with diabetes may experience nocturia, or frequent nighttime urination, and those with painful diseases such as cancer, may wake due to pain and discomfort. 
8. Medications & Drug Use: Many medications have the potential to disturb sleep as a side effect. Anti-depressants, stimulants, and even antihistamines can influence how well and how long you sleep. If you experience sleep disturbances as a result of your medication, it’s important that you communicate with your doctor early. 

Brain waves are electrical signals in the brain that can be observed by the EEG. Each state of sleep (including NREM and REM) has its own unique brain wave pattern which can be measured to determine how much time a person spends in that particular stage.

In the first stage of sleep, brain waves move in what is called Alpha waves, a relatively low-frequency wave that can be measured at about 8 to 13 Hz. These waves move more slowly than when we are awake, but more quickly than the successive phases.

In phase two, brain waves move in an even lower frequency, called Theta waves at about 4 to 8 Hz, however, high-frequency spikes called spindles are still common. In this phase, we are in a deeper sleep than in phase one.

In phase three, brain waves move in Delta waves, which are slower than phase two, at about .5 to 4 Hz. In this phase, we are in our most deep and restorative sleep, where we spend the longest amount of time.

During REM sleep, the brain waves are still at a Delta frequency. However, after about ten minutes, they tend to speed back up as we transition back into stage one of sleep and begin the cycle again.

The number of times your heart beats each minute is known as your heart rate.  When you are sleeping your heart rate, blood pressure and breathing automatically change to align with the demands of that stage of sleep.

In NREM phases, (1-3) your heart rate will slow to a resting rate. Depending on your age and other health factors, this is likely somewhere in the 60-100 beats per minute range. Once you transition into REM sleep, however, heart rate can vary largely depending on what you dream about and if you move around in your sleep. If you have dreams that frighten you or make you upset, your heart rate is likely to increase. 

Breath frequency is a measurement of how many breaths you take per minute and is determined by your metabolic rate. For a healthy adult, a normal breath frequency at rest is between 12 to 20 per minute. In children and older adults, the frequency is somewhat higher, reaching up to 30 in seniors and 60 in infants.

During NREM, breath frequency may fall even lower, and six to eight evenly spaced breaths per minute is considered normal for some adults. During REM, however, the breathing pattern becomes irregular and can be more rapid (up to 30 breaths per minute).

Blood pressure is the measure of the pressure of blood in the arteries, which decreases gradually as a person falls asleep. Blood pressure is lowest during NREM and gradually increases until the person enters REM. During this stage, heart rate and blood pressure increase because the brain is getting ready to combine all its senses in order to create a dream. This results in an overall increased demand for oxygen and energy that is supplied by an increase in heart rate and blood pressure.

Mattress quality has been shown to have an effect on sleep quality, which is related to general health. High-quality mattresses are able to support the body's natural alignment, reduce pressure points and overall ergonomically enhance posture for deeper, more restful sleep. The most important factor in mattress selection may be whether or not the mattress allows your body to sleep in its naturally designed position.

A study was conducted in 2014 at Northwestern University with 173 volunteers who underwent polysomnography, which is an intensive recording of one's brain waves, heart rate, breathing patterns, and muscle tension during sleep. Testers noted that participants using a high-quality mattress fell asleep faster and had less wake time during the night. These participants also reported that they felt more refreshed in the morning than those who used lower-quality mattresses.

For information on choosing a high-quality product, look at our definitive guide to buying a mattress. 

An adult human being requires between 7 and 9 hours of sleep per day on average. This number is subject to change based on age, sex, and medical conditions.

At birth, infants require between 14 to 17 hours of sleep per day while adults require 7 to 9 hours per day. Boys aged 6 to 12 years old require the most amount of sleep at 9 to 11 hours. Girls 6 to 12 years old require 9 to 10 hours of sleep while boys aged 13 to 17 require 8 to 9 hours. Studies have shown that the quantity of sleep in the elderly is affected by many factors, some beyond their control.

When healing from surgery or other serious medical conditions, humans need more sleep in what is called a stress recovery cycle.

Genetics and neurotransmitters such as serotonin, norepinephrine, and dopamine influence how much sleep a person gets.

Serotonin is thought to be responsible for feelings of well-being which can cause one to stay awake at night while norepinephrine contributes to the arousing effects on your brain when it releases in response to stress or excitement.

Dopamine is a hormone that stimulates the brain which can keep a person awake. Higher levels of dopamine have been related to insomnia in some studies.

Genes play a role in sleep as well by determining how much sleep an individual needs on average although psychiatric disorders such as depression and anxiety, as well as high blood pressure, high cholesterol, obesity, and diabetes are all known to affect sleep duration.

Sleep needs can change based on age and gender. One study entitled “Sleep in America” found that sleep need is related to age, gender, and genetic predisposition.

Infants up to three months old require between 14-17 hours of sleep per day. Babies between the ages of three and 11 months require 12-15 hours of sleep each day. Toddlers between the ages of one and two years old require 11-14 hours of sleep daily. Children between the ages of three and five require 10-13 hours daily. Children between six and 12 require 9-11 hours daily. Youths between the ages of 13 and 17 require 8-10 hours daily, and adults 18 to 64 require 7-9 hours per day.

Individuals between the ages of 65 and 74 require 7-8 hours per day, individuals between 75 and 85 years old need 7 hours each day, while those over the age of 85 have been shown to require only 6-7 sleep hours on average. Women reportedly sleep more than men at all stages in life except adolescence.

Sleep deprivation is the condition of not having enough sleep; it can be either chronic or acute. Sleep deprivation can be caused by a variety of sleep disorders, as well as mental health conditions, and other health issues. 

A chronic sleep-restricted state can cause fatigue, daytime drowsiness, and cognitive impairments as well as an increased risk for motor vehicle collisions and work-related accidents.

Conversely, a single acute sleep restriction may result in heightened alertness and an improved mood, while a partial sleep deprivation may produce euphoria and increased energy.

Hormones such as cortisol play a large part in how much our bodies require for sleep. The time at which cortisol secretion occurs is known to be associated with the internal circadian clock whose 24-hour period causes fluctuations in hormone levels throughout the day.

Insomnia is described as the difficulty in initiating or maintaining sleep, despite adequate opportunity and circumstances for sleep. It can manifest itself in both psychiatric disorders (e.g., depression) and physical problems (e.g., asthma). Insomnia may be of short duration (transient) or of long duration (chronic). The difficulty may be with falling asleep or staying asleep or waking too early.

Individuals suffering from insomnia have been shown to experience high levels of psychological distress which can increase the risk of developing depressive symptoms. Additionally, individuals with depression are more likely to suffer insomnia. Insomnia is also associated with reduced physical well-being and lower energy levels.

Insomnia can be short-term or long-term depending on its cause, however, both forms of insomnia should be treated in order to restore normal sleeping patterns.

All animals require sleep for the same reasons humans do, the most important being to rest, consolidate memories, and repair our bodies. In mammals, sleep occurs in multiple episodes of approximately 90 minutes each throughout a 24-hour cycle. This allows for maintenance of energy when food intake is low or absent and when internal body needs are high.

In reptiles, sleep also occurs in multiple episodes of deep sleep followed by REM sleep, however, these cycles occur much more regularly and quickly than in mammals. Brain activity during REM sleep also occurs in a different part of the brain than in humans.

Birds also sleep in phases like reptiles and mammals, however, during periods of slow-wave sleep, the birds remain in a state of semi-consciousness to stay aware of their surroundings and potential dangers.

It is common for animals that are often prey to larger predators to sleep less than their “safer” cousins. For example, horses sleep a mere three hours per day on average, while a sloth may sleep over 15 hours a day.

In animals, sleep is regulated by physiological mechanisms that cause tiredness, though other evolutionary factors can account for differences between species.

Sleep deprivation in animals can lead to death; therefore, many safety measures are taken with most animal studies such as the use of EEG and other monitoring tools to ensure safety.

Human beings need sleep for three reasons, surviving, growing, and a functioning nervous system. We’ll go into detail on each main function of sleep below. 

Human beings need sleep to survive. Sleep is an essential and irresistible process for humans. At an organ system level, things are broken down, energy is restored, toxins are removed, regenerative processes are carried out, inflammation is reduced and the brain uses this time to do housecleaning.

Sleep is the primary process responsible for Human Growth Hormone (HGH) production, which is responsible for tissue repair and regeneration. During the first years of life, children grow at an accelerated pace and sleep enhances growth and development through adolescence. HGH also plays a significant role in regulating our metabolism: the more we sleep, the higher our metabolic rate will be; this is why we tend to gain weight when we don't get enough sleep.

The first thing to go when we are lacking sleep is our ability to function at our best, which leads to lower quality of work and school performance. Our nervous system regulates processes such as heart rate, blood pressure, digestion, hormone balance, appetite control, metabolism and it also controls cognitive functions such as memory and concentration.

During sleep, there are changes that occur at all levels of the neuroaxis including the brain, brainstem, and hypothalamus. When we lack sleep, our nervous system is unable to complete those functions, and we are more likely to be irritable, impatient, less focused, and able to concentrate, which leads to slower reaction times than normal.