Sleep and Circadian Rhythms: Psychology of Consciousness

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Sleep and Circadian Rhythms

Circadian Rhythms

Circadian rhythms are biological processes that follow a roughly 24-hour cycle, regulating sleep and wakefulness in humans. These rhythms are controlled by specific brain structures and respond to both external and internal cues.

Circadian Rhythm: The natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours.
Suprachiasmatic Nucleus (SCN): A structure in the hypothalamus that acts as the master clock, controlling circadian rhythms. It responds primarily to light but can also react to hormonal, temperature, and eating cues.
Pineal Gland: Receives signals from the SCN and releases the hormone melatonin, which promotes sleep.
Melatonin: A hormone released by the pineal gland, increasing in darkness and signaling the body to prepare for sleep.
Dysregulation: Circadian rhythms can be disrupted by factors such as jetlag or working night shifts.

Example: When light decreases, the SCN triggers the pineal gland to release melatonin, leading to sleepiness.

Diagram of brain showing SCN, pineal gland, and eye pathway

Additional info: The SCN receives direct input from the eyes, allowing it to synchronize the body's rhythms with the external environment.

Sleep Cycle

The sleep cycle refers to the movement through different stages of sleep, which are divided into two general phases: REM and NREM. Each cycle lasts approximately 90-120 minutes, and people typically experience 4-6 cycles per night.

REM Sleep (Rapid Eye Movement): Characterized by rapid eye movements, vivid dreams, increased heart rate and respiration, and paralysis of skeletal muscles.
NREM Sleep (Non-Rapid Eye Movement): Subdivided into four stages, ranging from light to deep sleep, each distinguished by unique brainwave patterns.
Sleep Cycle: Each cycle includes NREM stages followed by a REM phase.
EEG (Electroencephalogram): Used to visualize brainwaves and measure electrical activity in the brain during sleep.

Sleep cycle diagram showing stages and REM

Example: REM sleep is associated with dreaming and muscle paralysis, while NREM sleep includes four distinct stages with varying brainwave activity.

EEG and Brainwaves

EEG is a tool used to measure and visualize the electrical activity of the brain. Two main properties are considered: frequency and amplitude.

Frequency: The number of wave cycles per second. High frequency indicates alertness or REM sleep, while low frequency is associated with deep sleep.
Amplitude: The height of the waves. High amplitude is seen in deep sleep, while low amplitude is typical of wakefulness and REM sleep.

Example: Beta waves (high frequency, low amplitude) are seen when awake; delta waves (low frequency, high amplitude) are seen in deep sleep.

Low frequency wave High frequency wave

Stages of Sleep

Sleep is divided into several stages, each with distinct EEG patterns and physiological characteristics.

Stage	Description	EEG Features
Awake	Fully conscious	Beta waves; high frequency, low amplitude
Stage 1	Light sleep	Theta waves; lower frequency than beta
Stage 2	Moderately deep sleep	Sleep spindles (very high frequency), K complexes (very high amplitude)
Stages 3 & 4	Deep sleep	Delta waves; lowest frequency, highest amplitude
REM	Dreaming, partial paralysis, increased heart rate	Resembles beta waves (similar to being awake)

EEG for Stage 1 sleep EEG for Stage 2 sleep EEG for Stages 3 & 4 sleep

Memory Tool: Busy beta waves (awake), tired theta waves (Stage 1), deep sleep delta waves (Stages 3 & 4).

Sleep Patterns Throughout the Night

As the night progresses, the amount of time spent in each stage of sleep changes. Deep sleep (Stages 3 & 4) is more prominent early in the night, while REM periods become longer toward morning.

Stages 3 & 4: Decrease in duration as the night progresses.
REM: Increases in duration as the night progresses.
Stage 1: Only re-entered if the person wakes up.

Sleep pattern graph showing stages across the night

Example: After leaving deep sleep, the next stage is typically REM; after waking up, the cycle restarts at Stage 1.

Key Terms and Concepts

Beta Waves: High frequency, low amplitude; seen when awake.
Theta Waves: Lower frequency than beta; seen in Stage 1 sleep.
Sleep Spindles: Short bursts of high frequency activity; seen in Stage 2.
K Complexes: Short bursts of high amplitude activity; seen in Stage 2.
Delta Waves: Lowest frequency, highest amplitude; seen in deep sleep (Stages 3 & 4).

Additional info: Sleep is essential for memory consolidation, physical restoration, and emotional regulation.