Physiology of sleep. Control of the sleep‐wake cycle. Biological rhythms.
Sleep is a naturally recurring state of altered consciousness, characterized by reduced responsiveness to external stimuli, decreased muscle tone, and specific patterns of brain activity. It is essential for physical and mental restoration, as well as for various cognitive processes.
Sleep Stages and Characteristics
Sleep is not a uniform state but progresses through a cyclical pattern of distinct stages, each with unique electrophysiological features and physiological changes:
- Non‐Rapid Eye Movement (NREM) Sleep:
- Stage 1 (N1): A transitional phase from wakefulness to sleep, marked by drowsiness, slowing of heart rate and breathing, and the appearance of theta waves on EEG.
- Stage 2 (N2): Deeper sleep characterized by sleep spindles and K-complexes on EEG. Muscle activity further decreases, and eye movements slow down.
- Stage 3 (N3): Slow-wave sleep (SWS) dominated by delta waves on EEG. The deepest stage of NREM sleep, associated with the lowest levels of brain activity and physiological functions.
- Rapid Eye Movement (REM) Sleep: A stage of sleep characterized by rapid eye movements, vivid dreaming, increased brain activity (resembling wakefulness) with beta waves on EEG and muscle atonia. Heart rate, breathing, and blood pressure fluctuate, and body temperature regulation is impaired.
Control of the Sleep‐Wake Cycle
The sleep-wake cycle is regulated by a complex interplay of neural circuits, neurotransmitter systems, and environmental cues.
- Circadian Rhythm: The suprachiasmatic nucleus (SCN) in the hypothalamus acts as the "master clock" for the circadian rhythm, which governs the roughly 24-hour cycle of sleep and wakefulness. The SCN receives input from the retina about light levels, synchronizing the circadian rhythm with the external day-night cycle.
- Homeostatic Regulation: Sleep pressure accumulates the longer we stay awake, increasing the drive to sleep. This process is influenced by factors like the accumulation of adenosine, prostaglandin D2, and cytokines.
- Neural Circuits:
- Ventrolateral Preoptic Nucleus (VLPO): The "sleep switch," located in the hypothalamus. When activated, it inhibits arousal centers in the brainstem and hypothalamus, promoting sleep.
- Arousal Centers: Include areas like the locus coeruleus (norepinephrine),
- raphe nuclei (serotonin), tuberomammillary nucleus (histamine), and lateral hypothalamus (orexin/hypocretin). These centers promote wakefulness and alertness.
- Neurotransmitters: Various neurotransmitters play critical roles in regulating sleepwake cycles and different sleep stages:
- GABA: The primary inhibitory neurotransmitter, crucial for promoting sleep, especially NREM sleep.
- Acetylcholine: Increases cortical activation, associated with REM sleep and wakefulness.
- Norepinephrine and Serotonin: Generally promote wakefulness but are also involved in modulating different sleep stages.
- Orexin/Hypocretin: Maintains wakefulness and regulates sleep-wake transitions.