Sleep and Biological Rhythms: Physiology, Stages, and Disorders

Lecture Preview

• A Physiological and Behavioral Description of Sleep

• Disorders of Sleep

• Why Do We Sleep?

• Physiological Mechanisms of Sleep and Waking

• Biological Clocks

Physiological and Biological Description of Sleep

Measurement Techniques

Understanding sleep physiology relies on specific measurement tools that record electrical activity in the body during sleep.

• Electromyogram (EMG): Measures electrical potential from electrodes placed on or in muscle. Used to detect muscle tone and activity.

• Electro-Oculogram (EOG): Measures electrical potential from the eyes, using electrodes placed on the skin around them. Detects eye movements, especially important for identifying REM sleep.

Stages of Sleep

Sleep is divided into several stages, each with distinct physiological and behavioral characteristics. These stages cycle throughout the night.

• Stage N1: Very Light Sleep

  • Transition from wakefulness to sleep.

  • Drifting in and out of sleep; easy to awaken.

  • Hypnagogic jerk: sudden muscle contractions, often accompanied by a sensation of falling.

• Stage N2: Light but Maintained Sleep

  • Brain activity slows further but is still relatively light.

  • Breathing and heart rate slow down.

  • Sleep is maintained; harder to awaken than in N1.

• Stage N3: Deep Sleep (Slow Wave Sleep)

  • Characterized by low-frequency (delta) brain waves.

  • Very difficult to awaken; highest arousal threshold.

  • Critical for physical restoration and recovery.

• Stage REM: Rapid Eye Movement Sleep

  • Very active brain activity, similar to wakefulness.

  • Dreaming occurs; eyes move rapidly under closed eyelids.

  • Body is temporarily paralyzed (muscle atonia) to prevent acting out dreams.

  • Important for memory consolidation and mood regulation.

Summary Table: Stages of Sleep

Stage	Key Features
N1 (Light Sleep)	Transition from wakefulness, easy to awaken, hypnagogic jerks
N2 (Light, Maintained Sleep)	Slowed brain activity, decreased heart rate and breathing, sleep spindles and K-complexes (brief bursts of activity on EEG)
N3 (Deep/Slow Wave Sleep)	Delta waves, very hard to awaken, physical restoration
REM Sleep	Rapid eye movements, vivid dreams, muscle paralysis, memory processing

Key Terms

• Hypnagogic jerk: A sudden, involuntary muscle contraction that occurs as a person is falling asleep, often accompanied by a sensation of falling.

• Sleep spindle: A burst of oscillatory brain activity visible on an EEG that occurs during N2 sleep, thought to play a role in memory consolidation.

• K-complex: A single large high-voltage spike of brain activity that occurs during N2 sleep, often in response to external stimuli.

• Delta waves: High amplitude, low frequency brain waves (

  • REM (Rapid Eye Movement) sleep: A sleep stage characterized by rapid eye movements, vivid dreams, and muscle atonia.

Example: Sleep Cycle Progression

During a typical night, a person cycles through N1, N2, N3, and REM stages multiple times. Early in the night, deep N3 sleep predominates, while REM periods become longer toward morning.

Additional info: The sleep cycle typically lasts about 90 minutes, and adults experience 4-6 cycles per night. Disruptions in these cycles can affect cognitive function, mood, and physical health.