Non-Rapid Eye Movement (NREM) sleep, which is divided into three stages, is a phase of sleep without rapid eye movements. The transition into NREM sleep is marked by a general shift in brain wave activity, characterized by a decrease in frequency and an increase in amplitude. Each stage within NREM sleep has its own distinct pattern of brain waves.

Non-Rapid Eye Movement (NREM) Sleep

REM sleep is a phase of sleep distinguished by the rapid movement of the eyes. Brain wave activity during this stage closely resembles that of an awake person, and it is the period when most dreaming takes place. A key feature of REM sleep is the paralysis of the body's voluntary muscle systems, although muscles essential for circulation and respiration remain active. This combination of high brain activity and lack of muscle tone is why REM sleep is often called 'paradoxical sleep'. Additionally, this sleep stage is understood to be involved in processes of learning and memory.

Rapid Eye Movement (REM) Sleep

Hypnograms show how a person passes through the various stages of sleep.

Hypnogram

Beta waves are the dominant brain wave pattern during wakefulness. Compared to the brain wave patterns observed during sleep, they are characterized by having the highest frequency (13–30 Hz) and lowest amplitude, and they also exhibit more variability.

Beta Waves

Sleep is broadly divided into two distinct types: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM) sleep. These categories are differentiated by physiological characteristics such as eye movement, brain wave patterns, and muscle tone. Understanding these differences is fundamental to sleep science.

Comparison of REM and Non-REM Sleep

Brain wave frequency refers to the number of brain waves that occur within a one-second interval. It is a key metric used to differentiate sleep stages and is measured in Hertz (Hz).

Brain Wave Frequency

Brain wave amplitude is the measure of the height of a brain wave. Along with frequency, it is a characteristic used to distinguish different stages of sleep and wakefulness.

Brain Wave Amplitude

An EEG-over-time graph can compare sleep-stage waveforms by showing wakefulness, Stage 1 NREM, Stage 2 NREM, Stage 3 NREM, and REM sleep on separate rows. In this example, awake activity has closely spaced waves with medium amplitude; Stage 1 NREM has relatively low-amplitude waves with periodic amplitude increases; Stage 2 NREM resembles Stage 1 but includes a K-complex around 10 to 12 seconds; Stage 3 NREM has a more uniform wave with gradually increasing amplitude; and REM resembles Stage 2 without the K-complex.

Visual Representation of Brainwaves in Sleep Stages

A sleep researcher observes a participant's brain wave activity. At one point in the night, the readout shows low-frequency, high-amplitude waves. Later, the readout shows high-frequency, low-amplitude waves, similar to an awake state, accompanied by rapid eye movements. What is the most fundamental conclusion that can be drawn from these distinct patterns?

Sleep is not a single, uniform state but is composed of several distinct stages. These stages are primarily differentiated by their unique patterns of brain wave activity, which vary in frequency and amplitude. The two main phases of sleep are Rapid Eye Movement (REM) sleep and non-REM (NREM) sleep.

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Sleep is a state of consciousness characterized by reduced physical activity and sensory awareness. Its regulation is governed by the interplay of two key mechanisms: circadian rhythms, which align sleep with the 24-hour day, and homeostatic processes, which build sleep pressure the longer one is awake.

Sleep

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Stages of Sleep

Dreams are a hallucinatory experience that can occur during sleep. Theories and analyses of dreams have been proposed by many psychologists and therapists throughout the centuries.

Dreams

Sleep disorders include:
- Parasomnia
- Insomnia
- Sleep Apnea
- Sudden Infant Death Syndrome (SIDS)
- Narcolepsy 

Sleep Problems and Disorders

A painting depicting two children asleep serves as a visual example of the state of sleep.

Example of Sleep in Art: Painting of Two Sleeping Children

The invention of electric light has contributed to a general decline in the amount of sleep people get. By illuminating the night, electric light enables more activity after dark than was possible for our ancestors, leading to reduced sleep durations.

Impact of Electric Light on Sleep Patterns

The National Sleep Foundation is an organization that provides research-based recommendations on sleep health, including guidelines for appropriate sleep durations for different age groups.

National Sleep Foundation

The required amount of sleep changes significantly throughout a person's life. While infants may sleep for as long as 16 hours daily, this duration progressively decreases with age. A comprehensive meta-analysis of multiple studies confirmed this trend, showing that by the age of 65, the average person sleeps for fewer than 7 hours per day.

Changes in Sleep Duration Across the Lifespan

Humans dedicate approximately one-third of their lives to sleeping. Based on an average life expectancy of 73 to 79 years, this amounts to an estimated 25 years spent asleep over a lifetime.

Proportion of Human Life Spent Sleeping

Sleep regulation refers to the brain's control over the timing and duration of sleep and wakefulness. This process is governed by the interaction of two primary systems: the circadian rhythm, which aligns sleep with the 24-hour environmental cycle, and homeostatic regulation, which builds sleep pressure based on the length of prior wakefulness.

Sleep Regulation

Sleep is closely linked with the secretion and regulation of several key hormones from various endocrine glands. This includes melatonin from the pineal gland, as well as follicle-stimulating hormone (FSH), luteinizing hormone (LH), and growth hormone from the pituitary gland.

Hormone Regulation During Sleep

While sleep is known to be essential, its precise function remains a scientific mystery. Despite this uncertainty, several prominent hypotheses have been proposed to explain why we sleep, which are often categorized by their focus on adaptive or cognitive benefits.

Hypotheses on the Function of Sleep

Sufficient sleep provides numerous advantages that contrast with the negative effects of sleep deprivation. According to sources like the National Sleep Foundation, these benefits include the maintenance of physical health, reduction of stress, mood enhancement, and improved motor coordination. Furthermore, adequate sleep is crucial for cognitive processes and the formation of memories.

Benefits of Adequate Sleep

The function and patterns of sleep are not universal across the animal kingdom; instead, they appear to be adaptations shaped by the distinct evolutionary pressures each species faces. This suggests that there may not be a single, overarching purpose for sleep. For instance, sleep requirements vary dramatically, with some fish not sleeping at all and giraffes needing very little, while for other species like rats, it is essential for survival.

Species-Specific Sleep Patterns as Evolutionary Adaptations

Sleep plays a crucial role in physical development, as several hormones essential for growth and maturation are secreted during sleep periods.

Role of Sleep in Hormone Secretion for Growth

Based on the case study below, use the two primary mechanisms of sleep regulation to explain the traveler's conflicting feelings of fatigue and wakefulness.

Analyzing Sleep Disruption

Sleep is different from simply resting while awake because sleep is a separate state marked by lower physical activity and reduced sensory awareness.

Sleep Versus Wakeful Rest

Sleep debt is the accumulated shortage that occurs when a person chronically gets less sleep than they need. Sleep debt can lower alertness and reduce mental efficiency.

Sleep Debt

In 2015, the National Sleep Foundation updated its sleep-duration recommendations to better reflect individual differences, grouping sleep durations into three categories: recommended, may be appropriate, and not recommended.

National Sleep Foundation's 2015 Sleep Duration Recommendations

Healthy sleep can be supported by keeping a consistent bedtime and wake time every day, avoiding stimulating activities and bright device light during the hour before bed, exercising daily, avoiding naps, keeping the bedroom cool between $$60$$ and $$67$$ degrees, avoiding alcohol, cigarettes, caffeine, and heavy meals before bed, and using the bed for sleep rather than studying, eating, or watching television. If sleep does not come easily, leaving the bed until tired again can help reinforce the bed as a cue for sleeping.

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