Circadian Rhythms - Body's Inner Clock
- Endogenous biological rhythms; period $\approx$ 24 hours.
- Master Pacemaker: Suprachiasmatic Nucleus (SCN) in hypothalamus.
- Input: Light (via Retinohypothalamic Tract (RHT) from ipRGCs with melanopsin).
- Intrinsic Period ($\tau$): $\approx$ 24.2 hours (free-running).
- Entrainment: Synchronization to zeitgebers (e.g., light-dark cycle - primary).
- Molecular: Clock genes (PER, CRY, CLOCK, BMAL1) feedback loops.
- Key Outputs:
- Melatonin (pineal): ↑ at night (darkness), sleep-promoting. Light inhibits.
- Cortisol (adrenal): ↑ in early morning, activity preparation.
- Disruption: Jet lag, shift work → health issues.
⭐ The Suprachiasmatic Nucleus (SCN) in the hypothalamus is the master circadian pacemaker.
Temperature Rhythm - Hot & Cold Cycles
Core body temperature (CBT) follows a daily rhythm, a key circadian marker.
- Nadir: Lowest point, ~2-4 AM (aids sleep).
- Peak: Highest point, ~4-6 PM (peak alertness).
- Sleep Onset: Requires CBT drop of ~0.5-1°C. 📌 Temp LOW when you're LOW (sleeping).
- SCN Control: Via autonomic nervous system & behavior.
- Influencers: Light, melatonin (↑ promotes ↓CBT), activity, meals.
| Time of Day | CBT Trend & Typical Value | Key Event(s) |
|---|---|---|
| Late Night/Early Morning (~2-4 AM) | ↓ Nadir (~36.2°C) | Deep Sleep, Max Melatonin |
| Morning | ↑ Rising | Waking, ↑Cortisol, Activity Start |
| Late Afternoon/Early Evening (~4-6 PM) | ↑ Peak (~37.2°C) | High Alertness, Post-Activity/Meals |
| Evening/Night | ↓ Falling | Sleep Initiation, Melatonin Rise |
⭐ The CBT nadir, occurring a few hours before waking, is a robust circadian phase marker; its timing shift indicates disorders like Delayed or Advanced Sleep-Wake Phase Disorder.
Activity Rhythms - Go, Rest, Repeat!
- Core: Regular cycles of activity/rest, vital for homeostasis; typically ~24-hour (circadian) period.
- Pacemaker: Suprachiasmatic Nucleus (SCN) in hypothalamus is primary.
- Rhythms persist without external cues (free-running), showing internal origin.
- Entrainment: Synchronization of internal rhythms to external environment.
- Zeitgebers ("Time Givers"): Cues entraining rhythms.
- Light-Dark Cycle: Dominant zeitgeber in humans.
- Others: Exercise, meal timing, social cues.
- Zeitgebers ("Time Givers"): Cues entraining rhythms.
- Assessment: Actigraphy (monitors movement).
- Examples: Sleep-wake cycle, hormone release patterns (e.g., cortisol), feeding.
- Clinical: Misalignment (jet lag, shift work) impacts health/performance. 📌 Remember "SCN synchronizes!"
⭐ Light is the most powerful zeitgeber for entraining human circadian rhythms, primarily acting via the retinohypothalamic tract (RHT) to reset the SCN.
Rhythm Disruptions - When Clocks Go Wrong
| Feature | Jet Lag | Shift Work Disorder (SWD) |
|---|---|---|
| Cause | Rapid travel across ≥2 time zones; Suprachiasmatic Nucleus (SCN) desynchronized from local cues. | Work schedule (e.g., night shifts) misaligned with internal body clock. |
| Symptoms | Insomnia/hypersomnia, daytime fatigue, GI upset, ↓cognition, malaise. | Insomnia or excessive sleepiness, fatigue, ↓alertness, ↑accident risk, mood issues. |
| Key Aspect | 📌 Eastward travel (phase advance) harder ("East is a bEast"); Westward (phase delay) easier ("West is Best"). Adaptation: ~1 day/timezone. | Chronic internal vs. external cue desynchronization. |
| Management | Timed melatonin (0.5-5 mg), bright light, strategic naps, short-acting hypnotics. | Optimized sleep hygiene, timed light/dark, melatonin (for daytime sleep), modafinil. |
⭐ Chronic circadian disruption (e.g., shift work) ↑ risk of cardiovascular disease, metabolic syndrome, & some cancers.
High‑Yield Points - ⚡ Biggest Takeaways
- Core body temperature (CBT) exhibits a robust circadian rhythm, nadir in early morning (e.g., 4 AM), peak in late afternoon.
- The SCN acts as the primary circadian pacemaker for temperature regulation.
- Melatonin release at night typically ↓ CBT, facilitating sleep onset.
- Activity levels are tightly linked to CBT; physical activity ↑ CBT.
- Fever can alter temperature set-points but the circadian oscillation often persists.
- Sleep-wake cycles and temperature rhythms are normally internally synchronized.
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