17.10 Sleep Architecture, Circadian Neurobiology, Sleep Disorders & Psychoactive Drugs

17.10 Sleep Architecture, Circadian Neurobiology, Sleep Disorders & Psychoactive Drugs

Sleep Architecture & Circadian Neurobiology

Circadian Rhythms & Neuroendocrine Regulation

Human biological rhythms adhere to a ~24-hour cycle synchronized by environmental zeitgebers (time-givers), primarily light:

1. Photons strike intrinsically photosensitive retinal ganglion cells (ipRGCs) expressing the photopigment melanopsin.
2. Signals propagate along the retinohypothalamic tract directly to the Suprachiasmatic Nucleus (SCN) of the anterior hypothalamus (the master biological circadian clock).
3. During daylight: SCN fires rapidly $\rightarrow$ inhibitory projection to the paraventricular nucleus $\rightarrow$ suppresses sympathetic activation of the pineal gland $\rightarrow$ inhibits the synthesis and secretion of melatonin.
4. At darkness: SCN firing diminishes $\rightarrow$ disinhibits the pineal gland $\rightarrow$ melatonin secretion into circulation surges, inducing somnolence.
5. Morning wakefulness is promoted by cortisol, released from the adrenal cortex via activation of the hypothalamic-pituitary-adrenal (HPA) axis (cortisol awakening response).

Electroencephalography (EEG) Waveforms

• Beta Waves ($\beta$): 13 to 30 Hz. High frequency, low amplitude, desynchronized. Characteristic of alert, active cognitive concentration.
• Alpha Waves ($\alpha$): 8 to 12 Hz. Synchronized. Characteristic of relaxed, awake states with eyes closed.
• Theta Waves ($\theta$): 4 to 7 Hz. Lower frequency, higher amplitude. Characteristic of light sleep (N1, N2) and drowsiness.
• Delta Waves ($\delta$): 0.5 to 4 Hz. Low frequency, high amplitude, highly synchronized. Characteristic of deep slow-wave sleep (N3).

The Four Stages of the Sleep Cycle

A normal night of sleep consists of 4 to 6 repeating 90-minute ultradian cycles, transitioning between Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM) sleep:

1. Stage 1 (NREM-1 / N1):
   • EEG: Transition from alpha to theta waves.
   • Physiology: Lightest sleep stage. Muscles relax, eyes roll slowly. Individuals experience hypnagogic hallucinations (vivid sensory flashes) and hypnic jerks (sudden myoclonic muscle twitches often accompanied by a sensation of falling).
2. Stage 2 (NREM-2 / N2):
   • EEG: Predominantly theta waves punctuated by two hallmark diagnostic waveforms:
     • Sleep Spindles: Bursts of 12 to 14 Hz rhythmic sinusoidal waves generated by thalamocortical oscillations. Essential for sleep maintenance and declarative memory consolidation.
     • K-Complexes: Isolated, high-amplitude biphasic complexes (sharp negative deflection followed by slower positive wave) evoked by external auditory stimuli or internal autonomic shifts, serving to protect sleep integrity.
   • Physiology: Heart rate slows, body temperature drops, eye movements cease.
3. Stage 3 (NREM-3 / N3 / Slow-Wave Sleep, SWS):
   • EEG: Dominated by low-frequency, high-voltage delta waves (>20% to >50% of the epoch).
   • Physiology: Deepest, most restorative stage of sleep. Heart rate, blood pressure, and metabolic rate reach lowest daily levels. Anterior pituitary releases pulses of Growth Hormone (GH), stimulating tissue repair and cellular regeneration. Essential for declarative memory consolidation.
   • Clinical Note: Parasomnias (sleepwalking, night terrors) occur during Stage 3 SWS.
4. REM Sleep (Rapid Eye Movement / "Paradoxical Sleep"):
   • EEG: Low-voltage, desynchronized, mixed-frequency waveform resembling alert wakefulness (sawtooth waves, beta/theta-like rhythm).
   • Physiology: Bursts of rapid conjugate eye movements; irregular respiration and heart rate; penile/clitoral tumescence.
   • Muscular Atonia: Complete paralysis of skeletal muscle groups (except extraocular muscles and diaphragm), driven by glycinergic and GABAergic inhibitory projections from the subcoeruleus nucleus in the pons to alpha motor neurons in the spinal cord. Prevents physical enactment of dreams.
   • Cognitive Role: Site of vivid, bizarre, narrative dreams; critical for emotional regulation, creativity, and procedural/implicit memory consolidation.

Ultradian Cycle Dynamics Across the Night

The proportion of sleep stages shifts across the night:

• First Half of Night: Dominated by prolonged periods of NREM-3 slow-wave sleep.
• Second Half of Night: Slow-wave sleep disappears; REM sleep episodes lengthen dramatically, dominating early morning hours.

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Neurophysiological Stages of Sleep

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Sleep Disorders: Dyssomnias versus Parasomnias

• Dyssomnias: Disorders affecting the quantity, quality, or timing of sleep:
  • Insomnia: Chronic difficulty initiating, maintaining, or returning to sleep.
  • Narcolepsy: Sudden, irresistible daytime sleep attacks. Characterized by cataplexy (sudden loss of muscle tone triggered by strong positive emotions like laughter), hypnagogic hallucinations upon falling asleep, sleep paralysis, and immediate intrusion of REM sleep within 10 minutes of sleep onset. Caused by autoimmune destruction of hypocretin/orexin-producing neurons in the lateral hypothalamus.
  • Sleep Apnea: Repeated pauses in breathing during sleep (>10 seconds), causing arterial hypoxia and frequent micro-arousals out of deep sleep:
    • Obstructive Sleep Apnea (OSA): Physical collapse of pharyngeal soft tissue during inspiration.
    • Central Sleep Apnea (CSA): Failure of medullary respiratory centers to send motor impulses to the diaphragm.
• Parasomnias: Abnormal behaviors, physiological events, or motor acts occurring during sleep:
  • Somnambulism (Sleepwalking): Motor ambulation occurring during NREM-3 slow-wave sleep. Individual has glassy eyes, is difficult to awaken, and has total morning amnesia.
  • Night Terrors (Pavor Nocturnus): Abrupt awakenings from NREM-3 slow-wave sleep accompanied by a terrified scream, intense autonomic sympathetic arousal (tachycardia, diaphoresis), and minimal dream recall.
  • REM Sleep Behavior Disorder (RBD): Failure of normal pontine glycinergic/GABAergic skeletal muscle atonia during REM sleep. Patients physically act out their violent dreams (kicking, punching), often injuring themselves or bed partners. Strongly predictive of developing alpha-synucleinopathies (Parkinson's disease, Lewy body dementia).

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Psychoactive Drug Classes & Neuropharmacology

All addictive substances activate the mesolimbic dopamine pathway (projecting from the ventral tegmental area (VTA) along the medial forebrain bundle to the nucleus accumbens):

1. Depressants:
   • Alcohol, Barbiturates, Benzodiazepines: Act as positive allosteric modulators of the $GABA_A$ receptor, a ligand-gated chloride ($Cl^-$) channel. Binding increases the frequency (benzodiazepines) or duration (barbiturates) of channel opening, causing intracellular $Cl^-$ influx and hyperpolarization, inducing generalized CNS depression. Alcohol also inhibits NMDA glutamate receptors.
2. Stimulants:
   • Amphetamines: Inhibit the vesicular monoamine transporter (VMAT2) and reverse dopamine transporters (DAT), pumping dopamine, norepinephrine, and serotonin directly into the synaptic cleft.
   • Cocaine: Blocks the reuptake of monoamines (dopamine, norepinephrine, serotonin) by competitively binding to DAT, NET, and SERT transporters.
   • Nicotine: Agonist at nicotinic acetylcholine receptors (nAChRs).
   • Caffeine: Competitive antagonist at adenosine receptors ($A_1$ and $A_{2A}$), preventing endogenous adenosine from inducing sedation.
3. Opioids & Opiates:
   • Morphine, Heroin, Oxycodone, Fentanyl: Agonists at G-protein coupled $\mu$-opioid receptors, mimicking endogenous endorphins. Inhibit adenylyl cyclase, close voltage-gated $Ca^{2+}$ channels, and open inwardly rectifying $K^+$ channels, suppressing nociceptive transmission and producing intense euphoria. Overdose causes fatal respiratory depression (hypoventilation due to blunted medullary chemoreceptor responsiveness), rapidly reversed by the competitive opioid antagonist naloxone.
4. Hallucinogens:
   • LSD, Psilocybin, Mescaline, DMT: Agonists or partial agonists at serotonin $5\text{-HT}_{2A}$ receptors, disrupting sensory gating in the thalamus, causing sensory distortion, synesthesia, and ego dissolution.