5 Stages Of Sleep In Psychology

[Nicol Allphin]

Sleepis important to a number of brain functions, including how nerve cells (neurons) communicate with each other. In fact, your brain and body stay remarkably active while you sleep. Recent findings suggest that sleep plays a housekeeping role that removes toxins in your brain that build up while you are awake.

Sleep is a complex and dynamic process that affects how you function in ways scientists are now beginning to understand. This booklet describes how your need for sleep is regulated and what happens in the brain during sleep.

The thalamus acts as a relay for information from the senses to the cerebral cortex (the covering of the brain that interprets and processes information from short- to long-term memory). During most stages of sleep, the thalamus becomes quiet, letting you tune out the external world. But during REM sleep, the thalamus is active, sending the cortex images, sounds, and other sensations that fill our dreams.

The basal forebrain, near the front and bottom of the brain, also promotes sleep and wakefulness, while part of the midbrain acts as an arousal system. Release of adenosine (a chemical by-product of cellular energy consumption) from cells in the basal forebrain and probably other regions supports your sleep drive. Caffeine counteracts sleepiness by blocking the actions of adenosine.

There are two basic types of sleep: rapid eye movement (REM) sleep and non-REM sleep (which has three different stages). Each is linked to specific brain waves and neuronal activity. You 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.

Stage 1 non-REM sleep is the changeover from wakefulness to sleep. During this short period (lasting several minutes) of relatively light sleep, your heartbeat, breathing, and eye movements slow, and your muscles relax with occasional twitches. Your brain waves begin to slow from their daytime wakefulness patterns.

Stage 2 non-REM sleep is a period of light sleep before you enter deeper sleep. Your heartbeat and breathing slow, and muscles relax even further. Your body temperature drops and eye movements stop. Brain wave activity slows but is marked by brief bursts of electrical activity. You spend more of your repeated sleep cycles in stage 2 sleep than in other sleep stages.

Stage 3 non-REM sleep is the period of deep sleep that you need to feel refreshed in the morning. It occurs in longer periods during the first half of the night. Your heartbeat and breathing slow to their lowest levels during sleep. Your muscles are relaxed and it may be difficult to awaken you. Brain waves become even slower.

REM sleep first occurs about 90 minutes after falling asleep. Your eyes move rapidly from side to side behind closed eyelids. Mixed frequency brain wave activity becomes closer to that seen in wakefulness. Your breathing becomes faster and irregular, and your heart rate and blood pressure increase to near waking levels. Most of your dreaming occurs during REM sleep, although some can also occur in non-REM sleep. Your arm and leg muscles become temporarily paralyzed, which prevents you from acting out your dreams. As you age, you sleep less of your time in REM sleep. Memory consolidation most likely requires both non-REM and REM sleep.

Sleep-wake homeostasis keeps track of your need for sleep. The homeostatic sleep drive reminds the body to sleep after a certain time and regulates sleep intensity. This sleep drive gets stronger every hour you are awake and causes you to sleep longer and more deeply after a period of sleep deprivation.

Factors that influence your sleep-wake needs include medical conditions, medications, stress, sleep environment, and what you eat and drink. Perhaps the greatest influence is the exposure to light. Specialized cells in the retinas of your eyes process light and tell the brain whether it is day or night and can advance or delay our sleep-wake cycle. Exposure to light can make it difficult to fall asleep and return to sleep when awakened.

Night shift workers often have trouble falling asleep when they go to bed, and also have trouble staying awake at work because their natural circadian rhythm and sleep-wake cycle is disrupted. In the case of jet lag, circadian rhythms become out of sync with the time of day when people fly to a different time zone, creating a mismatch between their internal clock and the actual clock.

Millions of people are using smartphone apps, bedside monitors, and wearable items (including bracelets, smart watches, and headbands) to informally collect and analyze data about their sleep. Smart technology can record sounds and movement during sleep, journal hours slept, and monitor heart beat and respiration. Using a companion app, data from some devices can be synced to a smartphone or tablet, or uploaded to a PC. Other apps and devices make white noise, produce light that stimulates melatonin production, and use gentle vibrations to help us sleep and wake.

Your health care provider may recommend a polysomnogram or other test to diagnose a sleep disorder. A polysomnogram typically involves spending the night at a sleep lab or sleep center. It records your breathing, oxygen levels, eye and limb movements, heart rate, and brain waves throughout the night. Your sleep is also video and audio recorded. The data can help a sleep specialist determine if you are reaching and proceeding properly through the various sleep stages. Results may be used to develop a treatment plan or determine if further tests are needed.

During wakefulness, certain neurons fire in our brains, making us aroused and alert. However, during sleep, these neural circuits are inhibited, our muscles are completely relaxed, and our body becomes completely inactive (Schwartz & Roth, 2008).

Though research reveals that it is much more complicated, the levels of consciousness can be thought of in three components: conscious, preconscious, and unconscious, as originally defined by Sigmund Freud.

A sleeping person, for example, is in a state of unconsciousness; so is an individual in a coma or someone who has fainted. The difference, however, is that a sleeping person can be aroused if the stimulus is strong enough (for example, shaking the person, shining a bright light, or making a loud noise).

But how do our bodies know when to go back and forth between sleep and wakefulness? Luckily, we all have an internal clock telling us when to do so. The circadian rhythm, our 24-hour clock, operates as our sleep/wake cycle.

During non-REM sleep, electrical activity in the brain slows, growth hormone secretion occurs, and there is a decrease in muscle activity, heart rate, respiration, and oxygen consumption (Purves et al., 2001).

This stage is heavily regulated by the brainstem (McCarley et al., 1995), which is the region of the brain that connects the cerebrum with the spinal cord. It consists of the midbrain, medulla oblongata, and pons.

During REM sleep, brain activity increases, voluntary muscles are inhibited, and rapid eye movements and dreams occur (McCarley et al., 1995). The next section will go into more detail on REM sleep as well as the characteristics of the specific stages of non-REM sleep.

In 1957, William Dement, along with Nathaniel Kleitman, came out with another revolutionary paper that discussed the specific sleep stages that together form the internal cycle that occurs every night we sleep (Dement & Kleitman, 1957).

In the beginning of stage 1, the brain produces high-amplitude alpha waves and begins to produce theta waves as the stage progresses (Abeln et al., 2014). Put simply; brainwaves are electrical pulses in the brain that change according to what we are doing or how we are feeling (Abhang et al., 2016).

Alpha waves are the highest frequency (thus, the fastest) of the three brain waves that characterize sleep, explaining why, when we have just fallen asleep and are not yet in a state of deep sleep, we can be easily awoken.

Stage 2 (N2) is still a period of light sleep, marked by similar characteristics as in N1, such as a continued slowing of both the heartbeat and breathing and the muscles relaxing even further than in N1 (Lockett, 2020).

Formerly known as stages 3 and 4, stage 3 (N3) is the final stage of non-REM sleep. During N3, your heartbeat and breathing slow to their lowest levels, and your muscles are so relaxed that it may be hard to awaken you (Lockett, 2020).

Sleepwalking, or somnambulism, occurs when sleeping and waking are combined, and the individual wanders around in a dazed and uncoordinated state (Kales et al., 1980). Night terrors are partial waking from sleep during which behaviors such as screaming, kicking, and panic occur (Kales et al., 1980).

EEG is an electrophysiological monitoring method to record the electrical activity of the brain, and this technology is commonly used in sleep studies to measure brainwave activity (Niedermeyer & da Silva, 2004).

At birth, humans spend about one-third of a 24-hour day in REM sleep. As we mature, the percentage of REM sleep declines rapidly so that by approximately age 10, the adult percentage of REM sleep is reached roughly 20-25% of total sleep time (McCarley et al., 1995).

However, REM rebound, or the increase of REM sleep above normal levels, can occur after a period of sleep deprivation (Verma et al., 2001). That is, there is an increase in REM sleep after a night of little REM sleep.

Evidence of dreams occurring during REM sleep is illustrated by studies in which subjects who are awakened from REM sleep recall elaborate, vivid, emotional dreams, as opposed to subjects awakened during non-REM sleep who report fewer, less vivid dreams (Purves et al., 2001).

Humans may be unique in a lot of ways, but the fact that we sleep is not one of them. As discussed, sleep is important for recovery, memory storage, and growth, so it makes perfect sense that other animals need sleep, too.

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