Sleep (College Board AP® Psychology): Study Guide

What is sleep?

• Sleep is a naturally occurring altered state of consciousness

  • It is characterized by reduced awareness and responsiveness to the environment

• Sleep is not a uniform state

  • It consists of distinct stages that cycle repeatedly throughout the night, each identified by characteristic brain wave patterns recorded using an EEG

The circadian rhythm

• The circadian rhythm is the body's internal biological clock that regulates the sleep/wake cycle across a roughly 24-hour period

  • It is influenced by environmental cues, e.g. light levels, which signal to the brain when to promote wakefulness and when to promote sleep

• The suprachiasmatic nucleus (SCN) in the hypothalamus coordinates the circadian rhythm, by regulating the release of melatonin from the pineal gland in response to light levels

  • As light decreases in the evening, melatonin levels rise, promoting sleepiness

  • As light increases in the morning, melatonin levels fall, promoting wakefulness

• Disruptions to the circadian rhythm have measurable effects on behavior and mental processes

  • Jet lag occurs when rapid travel across time zones causes a mismatch between the body's internal clock and the external environment

    • The result is fatigue, impaired concentration, and disrupted sleep

  • Shift work involves working hours that conflict with the body's natural sleep/wake cycle

    • This can cause chronic disruption that is associated with sleep disorders, cognitive impairment, and long-term health consequences

Cultural and contextual influences

• Cultural norms and expectations significantly influence sleep behavior

  • What is considered a normal sleep schedule, acceptable sleep duration, and appropriate times for sleep varies across cultures

• For example:

  • Afternoon napping (siesta) is a culturally accepted practice in some Mediterranean and Latin American cultures

    • This reflects a natural dip in the circadian rhythm in the early afternoon

  • Many Western industrial cultures prioritize continuous nighttime sleep and treat daytime napping as a sign of laziness or poor health, despite evidence that it may be beneficial

• Cognitive biases also play a role

  • Many people underestimate the impact of sleep deprivation on their performance, believing they have adapted to reduced sleep when cognitive impairment persists

The stages of sleep

• Sleep is divided into two main types:

  • NREM (non-rapid eye movement) sleep

  • REM (rapid eye movement) sleep

• Each stage produces a distinct EEG pattern

  • This allows researchers to identify sleep stages objectively

• A full sleep cycle lasts approximately 90 minutes and repeats four to six times per night, with the proportion of REM sleep increasing across cycles

NREM sleep (Stages 1–3)

• Hypnagogic sensations are vivid sensory experiences that occur as a person transitions into Stage 1 sleep

  • They may include the sensation of falling, brief visual images, or muscle twitches (hypnic jerks)

• NREM Stage 3 is the deepest stage of sleep and is most important for physical restoration and immune function

• The duration of NREM sleep, particularly Stage 3, decreases across the night as REM sleep becomes progressively longer

REM sleep

• REM sleep is an active stage of sleep during which the brain produces wave patterns similar to those seen during wakefulness

• REM sleep is described as paradoxical because brain activity is high but the body is at its most physically relaxed

  • Voluntary muscles are temporarily paralyzed

  • Dreaming most commonly occurs during REM sleep

• The frequency and duration of REM sleep increases as the night progresses

  • Early cycles contain short REM periods while later cycles contain longer ones

• REM rebound occurs when a person is deprived of REM sleep

  • On subsequent nights, the body compensates by spending a greater proportion of sleep time in REM, producing more vivid and frequent dreams

Why do we sleep?

• Two current theories explain the function of sleep in terms of cognitive and physiological restoration:

  • Memory consolidation theory

  • Restoration theory

Memory consolidation theory

• Memory consolidation theory proposes that sleep, particularly REM sleep, is critical for:

  • organizing memories

  • consolidating memories

  • strengthening memories

• During sleep, the brain replays and integrates newly encoded information, transferring it from short-term to long-term memory storage

  • Research shows that people who sleep after learning perform significantly better on memory tests than those who remain awake

Restoration theory

• Restoration theory proposes that sleep allows the body and brain to restore depleted resources used during waking activity

• During deep NREM sleep, the body:

  • repairs tissue

  • synthesizes proteins

  • releases growth hormone

• Sleep deprivation produces measurable cognitive and physical impairment, supporting the view that sleep serves a restorative function