Understanding Sleep Stages

Sleep is an important part of our life. Quality sleep – and getting enough of it at the right times — is essential.  Without adequate sleep it’s harder to concentrate and respond quickly. Sleep also affects almost every type of tissue and system in the body including brain, heart, lungs, metabolism, immune function, mood, and disease resistance.  Research has shown that a lack of sleep, or getting poor quality sleep, increases the risk of disorders including high blood pressure, cardiovascular disease, diabetes, depression, and obesity.

The majority of people will just consider how many hours of sleep we require. While sleep duration is certainly significant, it isn’t the only factor to consider. It’s also important to consider the quality of sleep and whether or not the time spent sleeping is genuinely restorative. A critical aspect of receiving truly high-quality rest is progressing smoothly through the sleep cycle, which is made up of four distinct sleep stages. Each stage of sleep contributes to your mind and body waking up refreshed. Understanding the sleep cycle also aids in explaining how sleep problems such as insomnia and obstructive sleep apnea can affect a person’s sleep and overall health.

Sleep Cycle

Sleep isn’t always consistent. Instead, entire sleep is made up of numerous rounds of sleep cycle, each of which is made up of four separate stages, over the course of the night. A person passes through four to six sleep cycles in a typical night. Although not all sleep cycles have the same length, they usually last around 90 minutes.

As one proceeds through nighttime sleep, it’s typical for sleep cycles to change. The initial sleep cycle is usually the shortest, lasting between 70 and 100 minutes, while subsequent cycles last between 90 and 120 minutes. Furthermore, as the night progresses, the amount of time spent in each sleep stage also alters.

Sleep cycles differ from person to person and night to night depending on a variety of factors, including age, previous sleep patterns, and alcohol usage etc.

Sleep Stages

Sleep can be broadly segmented into two basic types of sleep:  rapid eye movement (REM) sleep and non-REM sleep (which has three different stages), thus there are 4 stages of sleep.

REM Sleep

Rapid eye movement sleep (REM sleep or REMS) is a unique phase of sleep in mammals and birds, characterized by random rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vividly.

Non-REM Sleep (Stages 1 – 3)

Non-rapid eye movement sleep (NREM), also known as quiescent sleep, is, collectively, sleep stages 1–3. There are distinct electroencephalographic and other characteristics seen in each stage. Unlike REM sleep, there is usually little or no eye movement during these stages. Dreaming occurs during both sleep states, and muscles are not paralyzed as in REM sleep.

Each sleep stage is linked to specific brain waves and neuronal activity.  It is normal to cycle through all stages of non-REM and REM sleep several times during a typical night, with increasingly longer, deeper REM periods occurring toward morning.

The breakdown of a person’s sleep into various cycles and stages is commonly referred to as sleep architecture. A hypnogram can be used to visualize the sleep architecture of someone who has had a sleep study.

Stage 1

Non-REM sleep stage 1 is the changeover from wakefulness to sleep.  It is essentially the “dozing off” stage, and it normally lasts just one to five minutes. During this short period of relatively light sleep, heartbeat, breathing, and eye movements slow down, and muscles relax with occasional twitches.  Brain waves begin to slow from their daytime wakefulness patterns.

It’s easy to wake someone up during stage 1 of sleep, but if not disturbed, the person can swiftly progress to stage 2. As the night progresses, an undisturbed sleeper may spend less time in stage 1.

Stage 2

Non-REM sleep stage 2 is a period of light sleep before a person enters deeper sleep.  The heartbeat and breathing slow, and muscles relax even more.  Body temperature drops and eye movements stop.  Brain wave activity slows down but is marked by brief bursts of electrical activity.  A person spends more time of repeated sleep cycles in sleep stage 2 than in other sleep stages.

During the first sleep cycle, stage 2 sleep can last anywhere from 10 to 25 minutes, and each stage 2 in subsequent sleep cycles can get longer during the night. A person normally spends roughly half of his or her sleep time in sleep stage 2.

Stage 3

Non-REM sleep stage 3 is the period of deep sleep that one needs to feel refreshed in the morning. It occurs in longer periods during the first half of the night.  The heartbeat and breathing slow down to their lowest levels during sleep stage 3.  Muscles are relaxed and it may be difficult to awaken the sleeping person.

Brain waves become even slower. Brain waves during this sleep stage are of a recognizable pattern and are known as delta sleep or slow-wave sleep (SWS).

This stage, according to experts, is crucial to restorative sleep because it allows for physical regeneration and growth. It could also help the immune system and other vital biological functions. Despite the fact that brain activity is lowered, deep sleep has been shown to aid with perceptive thinking, creativity, and memory.

Sleep stages 3 typically last 20-40 minutes during early sleep cycles. As the sleep becomes longer, these non-REM stages get shorter, and more time is spent in REM sleep.

REM Sleep (Stage 4)

First REM-sleep occurs about 90 minutes after falling asleep.  Eyes move rapidly from side to side behind closed eyelids.  Mixed frequency brain wave activity becomes closer to that seen in wakefulness.  Breathing becomes faster and irregular, and heart rate and blood pressure increase to near waking levels.  Most of our dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Arm and leg muscles become temporarily paralyzed, which prevents the person from acting out the dreams.

In the opinion of sleep experts, REM sleep is thought to be necessary for cognitive functions such as memory, learning, and creativity. The considerable increase in brain activity during REM sleep is responsible for the most vivid dreams.

Under normal conditions, a person doesn’t enter a REM sleep stage until been asleep for about 90 minutes. As the night progresses, REM stages lengthen, especially in the second half of the night. While the first REM stage sleep may last only a few minutes, later stages can last for around an hour. In total, REM stages account for around 25% of sleep in adults.

With age, people sleep less time in REM sleep stage.

 Why Do Sleep Stages Matter?

Sleep stages are critical because they allow the brain and body to recover and develop. Insufficient sleep may have effects on thinking, emotions, and physical health, if both deep and REM sleep are not obtained. Memory consolidation most likely requires both non-REM and REM sleep.

Sleepers who are regularly disturbed during early stages of sleep, such as those suffering from sleep apnea, may find it difficult to cycle into these deeper stages. People who suffer from insomnia may not obtain enough overall sleep to complete each stage.

What Affects Sleep Stages?

There can be substantial individual variations based on a number of factors:

• Age: Over the course of a person’s life, the amount of time spent in each sleep stage fluctuates considerably. Newborns spend significantly more time in REM sleep (about 50%) and may enter it as soon as they fall asleep. As a child grows older, sleep patterns begin to resemble those of adults, with most children attaining a comparable sleep architecture by the age of five. Elderly people, on the other hand, spend less time in REM sleep.
• Alcohol: Early in the night, alcohol reduces REM sleep, but when the alcohol wears off, there is a REM sleep rebound, with longer REM stages.
• Drugs: Some drugs also can alter sleep architecture.
• Sleep Disorders: Multiple awakenings caused by sleep apnea, Restless Leg Syndrome (RLS), and other problems can disrupt a healthy sleep cycle.
• Sleep Patterns: An aberrant, irregular, sleep cycle can develop when a person obtains inconsistent or insufficient sleep for several days or more.

What to Do for a Healthier Sleep Cycle?

A person can take steps to improve chances of having a healthy progression through each sleep stage.

The key step is focusing on improving sleep hygiene, which relates to sleeping environment (comfortable mattress, pillows, and linens, etc.).

Consistent sleep schedules, and sleep-related routines, getting natural daylight exposure, avoiding alcohol before bedtime, reducing noise and light interruptions can all help a person get more restful sleep and encourage optimal circadian rhythm alignment.

If a sleep disorder like sleep apnea is suspected, then it’s important to talk with a doctor who can most appropriately guide for effective solution.

Exercising regularly and getting proper nutrition daily can help to get good quality sleep. Vigorous exercise may even help promote deep sleep.

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