Circadian Rhythms and Psychiatry

Core Concepts - Body's Master Clock

• Endogenous, self-sustaining ~24-hour cycles regulating sleep-wake, hormones, metabolism.
• Suprachiasmatic Nucleus (SCN): Master pacemaker in anterior hypothalamus.
  • Receives direct light input from intrinsically photosensitive retinal ganglion cells (ipRGCs) via retinohypothalamic tract (RHT).
  • Synchronizes peripheral "slave" oscillators in other tissues.
• Key Hormones:
  • Melatonin: Secreted by pineal gland in darkness; promotes sleep. Light suppresses its release.
  • Cortisol: Adrenal hormone; peaks in early morning, promoting alertness.
• Clock Genes: PER, CRY, CLOCK, BMAL1 form core transcriptional-translational feedback loops (TTFLs) driving cellular rhythms.

⭐ The SCN primarily uses GABA as its neurotransmitter for both internal communication and output signaling to other brain regions.

Regulation & Measurement - Keeping in Sync

• Regulation (Entrainment):
  • SCN (hypothalamus): Master biological clock.
  • Entrained by Zeitgebers (external cues).
  • Light: Most potent zeitgeber.
    • Pathway: Light → ipRGCs (retina, melanopsin) → Retinohypothalamic Tract (RHT) → SCN.
  • Other zeitgebers: Social cues, meal timing, physical activity, temperature.
• Measurement:
  • Actigraphy: Wrist-worn device; measures motor activity to estimate sleep-wake cycles.
  • Dim Light Melatonin Onset (DLMO): Gold standard for assessing circadian phase.
    • Serial saliva/plasma melatonin sampling under dim light (<10 lux).

    ⭐ DLMO typically occurs 2-3 hours prior to habitual sleep onset, marking the opening of the "sleep gate".

  • Core Body Temperature (CBT): Rhythm with nadir during late sleep/early morning.
  • Sleep Diaries: Subjective patient logs. ​

Psychiatric Links - When Rhythms Stumble

• Depression: Often phase-delayed (later sleep/wake). Altered melatonin & cortisol rhythms.
  • Sleep disturbances: Insomnia, hypersomnia.
• Bipolar Disorder: Extreme sensitivity to rhythm changes.
  • Mania: ↓ sleep needs, often phase-advanced.
  • Depression: Hypersomnia, phase-delayed.

  ⭐ Sleep deprivation can trigger manic episodes in bipolar disorder.

• Schizophrenia: Significant disruption of sleep-wake cycles & activity.
  • Clock gene associations (e.g., PER3, CLOCK).
• Anxiety Disorders: High rates of sleep disturbances.
  • Potential HPA axis & circadian interaction.
• ADHD: Common delayed sleep-wake phase disorder (DSWPD).
  • Melatonin often used for sleep onset.
• Therapeutic Target: Chronotherapies (light therapy, melatonin) show promise. oka

Chronotherapeutics - Timing for Wellness

• Light Therapy (Bright Light Therapy - BLT):
  • Exposure: 10,000 lux light, typically 30 min/day (morning).
  • Indications: SAD (Seasonal Affective Disorder), non-seasonal depression, sleep phase disorders.
  • Mechanism: Resets Suprachiasmatic Nucleus (SCN), ↓ melatonin.
• Melatonin:
  • Regulates sleep-wake; Dose: 0.5-5 mg at bedtime (hs).
  • Indications: Insomnia, jet lag, Delayed Sleep-Wake Phase Disorder (DSWPD).
  • Ramelteon: Selective MT1/MT2 receptor agonist.
• Agomelatine:
  • MT1/MT2 agonist & 5-HT2C antagonist.
  • Antidepressant; resynchronizes circadian rhythms.
  • Dose: 25-50 mg daily; ⚠️ Monitor Liver Function Tests (LFTs).
• Behavioral Strategies:
  • Sleep hygiene education, scheduled sleep-wake times.
  • Timed exercise, meal intake.
  • Social Rhythm Therapy (SRT) for bipolar disorder.

⭐ Light therapy (10,000 lux) is a first-line treatment for Seasonal Affective Disorder (SAD).

High-Yield Points - ⚡ Biggest Takeaways

• SCN (Suprachiasmatic Nucleus) in hypothalamus: the master circadian clock.
• Melatonin (pineal gland) regulates sleep; light exposure suppresses its release.
• Circadian disruptions are prominent in mood disorders (e.g., depression, bipolar disorder).
• DSWPD: Delayed sleep onset/offset ("night owls").
• ASWPD: Advanced sleep onset/offset ("morning larks").
• Light therapy: key treatment for SAD and certain circadian rhythm sleep disorders.
• Clock genes (e.g., PER, CRY, BMAL1) underpin circadian rhythmicity.