Circadian rhythm, sleep, and induced states of altered consciousness

The circadian rhythm is a 24-hour biological cycle controlled by the suprachiasmatic nucleus that governs sleep patterns, hormone production, and body temperature. Sleep is a physiologically recurring state of rest characterized by altered consciousness and inaction of voluntary muscles. Sleep usually consists of 4–5 sleep cycles that include a non-rapid eye movement sleep phase (NREM sleep) and a rapid eye movement sleep (REM sleep) phase. NREM sleep is divided into three stages of progressively deeper sleep, with stage N3 being crucial for physical restoration. In contrast, REM sleep is characterized by vivid dreaming, muscle atonia, and an EEG pattern resembling the waking state, and is thought to be important for memory consolidation. Sleep architecture changes with age, as the proportion of deep sleep and REM sleep decreases over a lifetime. Various sleep disorders are associated with specific sleep stages, such as sleepwalking during N3 sleep and nightmare disorder during REM sleep. Other induced states of altered consciousness include hypnosis, which is characterized by heightened suggestibility and focused attention, and meditation, a practice used to train awareness to achieve mental clarity. The different states of consciousness are associated with distinct EEG patterns, such as delta waves in deep sleep, beta waves in REM sleep, and alpha and theta waves in meditation.

Clinical content on sleep disorders, narcolepsy, restless legs syndrome, and obstructive sleep apnea is discussed in separate articles.

Definition: a 24-hour cycle of biophysical changes that regulate sleep patterns, feeding patterns, hormone production (e.g., release of melatonin, prolactin, ACTH, norepinephrine, growth hormone), and body temperature

• Involved pacemakers
  • Endogenous pacemaker (= internal clock): suprachiasmatic nucleus (SCN) of the hypothalamus
  • Exogenous zeitgebers (= external time cues): light (the most important cue), meals, social activity
• Regulation of sleep (melatonin pathway)
  • When light is present: activation of the photosensitive melanopsin-containing retinal ganglion cells (mRGCs) → stimulation of the retinohypothalamic tract (RHT) → stimulation of the suprachiasmatic nucleus (SCN) of the hypothalamus → inhibition of the sympathetic pathway to the pineal gland → melatonin release is suppressed
  • When light is absent: no activation of the mRGCs → stimulation of the RHT ceases → SCN is no longer stimulated by light and picks up its intrinsic circadian pacemaker activity → sympathetic pathway is disinhibited → norepinephrine release (from nerve endings at the pineal gland) → melatonin release from the pineal gland → sleep induction
• Regulation of wakefulness (cortisol pathway): internal SCN pacemaker activation at night → hypothalamus releases corticotropin-releasing hormone (CRH) → pituitary gland releases ACTH → adrenal gland releases cortisol → cortisol levels peak (approx. 6–8 AM), promoting wakefulness and energy mobilization → gradual decrease throughout the day → nadir (lowest point) in the middle of the night (approx. midnight)

SCN Lesion: Complete loss of circadian rhythm (sleep occurs in random bursts throughout day and night).

SCN = Sun-Censing Nucleus

Melatonin secretion is inhibited by light and peaks at night. Cortisol levels are lowest around midnight and peak just before awakening (6 AM–8 AM).↓

Definitions

• Sleep: a physiologically recurring state of rest characterized by altered consciousness and inaction of voluntary muscles
• Sleep latency: the time required to fall asleep
• Sleep cycles: A full night's rest typically consists of 4–5 sleep cycles of 90–120 minutes each.
• Sleep phases: Each sleep cycle consists of a non-REM sleep phase and REM sleep phase (so-called paradoxical sleep).
• Sleep stages: distinct periods of sleep characterized by specific EEG patterns and physiological changes

Sleep cycles generally follow an N1 → N2 → N3 → N2 → REM pattern.

Deep sleep (stage N3) occurs predominantly during the first half of the night. In contrast, REM sleep episodes increase in duration and frequency during the later hours of the sleep cycle, occurring most heavily just before waking.

Total sleep deprivation initially leads to decreased concentration and attention. If it persists, microsleep episodes can occur. Selective REM sleep deprivation can lead to irritability and difficulty with complex tasks!

Overview of sleep phases and stages ^[1][2]

“For BETTER (read “beta”) WAVES, Ask The Silent Surfer Dozing at the Beach:” Beta waves while awake; Alpha waves with eye closure; Theta waves during N1; Sleep spindles during N2; Delta waves during N3; Beta waves during REM sleep).

Theories of sleep function

• Restoration theory
  • Sleep is an essential process for repairing and restoring the body and brain from daily exertions.
  • NREM sleep: associated with physical restoration, e.g., tissue repair and release of growth hormone
  • REM sleep: associated with mental restoration and brain recovery
• Evolutionary theory (adaptive/inactivity theory)
  • Sleep is an adaptive behavior to conserve energy and reduce exposure to predators during less active or more dangerous periods (e.g., night).
  • Enhances survival and evolutionary success
• Information consolidation theory (brain plasticity theory)
  • Sleep supports key cognitive functions, including memory consolidation and learning.
  • NREM stage 2 (sleep spindles): associated with motor skill learning
  • Slow-wave sleep (N3): linked to declarative memory consolidation
  • REM sleep: associated with procedural memory and creative problem-solving
• Glymphatic system theory
  • Sleep serves to clear neurotoxic waste products from the central nervous system.
  • The glymphatic system utilizes cerebrospinal fluid (CSF) to "flush" metabolites (e.g., amyloid-beta) from the brain's interstitium.
  • This clearance system is significantly more active during sleep compared to wakefulness.
  • Clinical relevance: Impaired glymphatic function during sleep disruption may be linked to neurodegenerative disorders (e.g., Alzheimer disease).

Theories of dreaming

• Freud's wish-fulfillment theory (psychoanalytic theory)
  • Dreams are a gateway to the unconscious mind and function to fulfill unconscious wishes, desires, and unresolved conflicts.
  • Dreams contain two types of content:
    • Manifest content: the actual storyline and imagery of the dream (what is remembered)
    • Latent content: the hidden, symbolic, and psychological meaning behind the manifest content
• Activation-synthesis hypothesis (neurobiological theory)
  • Dreams are the brain's subjective interpretation of random neural firings originating in the brainstem (specifically the pons) during REM sleep.
  • The cerebral cortex receives these random signals and attempts to synthesize them into a coherent story or narrative.
  • This synthesis process is thought to explain the often bizarre, illogical, and disjointed nature of dreams.

Sleep over the course of life

• Total sleep duration decreases with increasing age: Newborns sleep about 16–18 hours per day!
• The proportion of REM sleep in total sleep decreases with increasing age.
• The proportion of N3 (deep) sleep also decreases with increasing age.

Overview of sleep-phase-specific disorders

“I saw the t(w)ooth fairy fleeing and the three little pigs peeing:” teeth grinding occurs during N2 stage and bedwetting during N3 stage NREM sleep.

• Definition: an induced state of consciousness characterized by heightened suggestibility, deep relaxation, and intense, focused attention
• Characteristics
  • Increased ability to respond to suggestions
  • Diminished attention to peripheral stimuli
• Induction: requires a rhythmic, monotonous stimulus (voice or visual) to shift focus from the external environment to the internal mental states
• EEG findings: associated with alpha waves (8-13 Hz}, which are characteristic of relaxed wakefulness (similar to light meditation or daydreaming)
• Theories of hypnosis
  • Dissociation theory (divided consciousness)
    • Hypnosis is a genuine altered state of consciousness.
    • Consciousness splits (dissociates) into two separate streams of awareness.
    • Example: Hypnotized individuals placing an arm in ice water may report no pain, following the hypnotist's suggestion. However, a "hidden" part of their awareness passively observes and can report the pain if prompted ("hidden observer" effect).
  • Social influence theory (social-cognitive theory)
    • Hypnosis is a social phenomenon and not a special altered state of consciousness.
    • Individuals actively play a hypnotized person and fulfill the expectations of the hypnotist.
    • Their behavior is a result of their own cognitive expectations and their desire to respond to social cues/be a good "subject".
    • Example: Hypnotized individuals placing an arm in ice water may report no pain because they believe that is what the situation requires.
• Applications: may be used in therapy for pain management, anxiety reduction, or behavior modification (e.g., smoking cessation)

The EEG pattern in hypnosis is primarily characterized by alpha waves (relaxed wakefulness).

• Definition: a practice that utilizes techniques such as focused breathing and guided imagery to train attention and awareness, often to achieve a state of mental clarity and emotional and physical calm
• Common forms
  • Focused attention (concentrative): attention is focused on a single object, such as the breath, a sound, or a mantra
  • Open-monitoring (mindfulness): involves non-judgmental awareness of all thoughts, feelings, and sensations as they arise
• EEG findings: associated with increased alpha and theta wave activity, depending on the meditative state and expertise of the practitioner
• Applications: may be used to help manage anxiety, depression, and chronic pain (e.g., with mindfulness-based stress reduction)