Physiology
Biological Clock
Internal timekeeping — circadian rhythms regulate sleep, metabolism, behavior
A biological clock is an internal timekeeping mechanism in organisms that regulates daily (circadian) rhythms — sleep/wake, metabolism, hormone release, body temperature. ~24-hour cycle. Master clock: suprachiasmatic nucleus (SCN) in brain hypothalamus; entrained to light by retinal signals. Cellular clocks: feedback loops of clock genes (Per, Cry, Bmal1, Clock). Disruption: jet lag, shift work, seasonal affective disorder. Discovered by experiments on plants in 1700s; circadian biology accelerated 1970s+. Nobel Prize 2017 (Hall, Rosbash, Young) for circadian gene discovery.
- Period~24 hours (circadian)
- Master clockSuprachiasmatic nucleus (SCN) in hypothalamus
- Light entrainmentRetinal photoreceptors signal SCN
- Clock genesPer, Cry, Bmal1, Clock (mammals)
- Nobel Prize 2017Hall, Rosbash, Young (circadian genes)
- DisruptionsJet lag, shift work, depression
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Why biological clock matters
- Sleep. Circadian regulates sleep/wake.
- Health. Disruption linked to disease.
- Drug timing. Some drugs work better at certain times.
- Jet lag. Common travel issue.
- Mental health. Depression, bipolar, seasonal affective.
- Athletic performance. Optimum varies by time of day.
- Industry. Shift work health effects.
Common misconceptions
- Clock is exactly 24 hours. Slight individual variation.
- Clock just from environment. Endogenous; entrained to environment.
- Disruption is harmless. Long-term health consequences.
- Only sleep affected. Many physiological processes.
- Adjust quickly. ~1-2 hours per day; week for major shifts.
- Single clock in body. Multiple clocks; SCN coordinates.
Frequently asked questions
How do circadian rhythms work?
~24-hour internal cycle. Endogenous (continues without external cues) but normally entrained to environment (especially light/dark). Master clock in SCN coordinates body. Cell clocks: gene expression feedback loops. Per/Cry proteins accumulate, inhibit own gene expression; degraded; cycle resets. ~24 hours per cycle. Synchronized across cells.
What's the SCN?
Suprachiasmatic nucleus. Pair of small structures (~20,000 cells each) in hypothalamus, above optic chiasm. Receives light signals from retina (specifically melanopsin-expressing ipRGCs — non-image-forming photoreceptors). Coordinates body clocks via: hormones (melatonin), neural signals, body temperature. SCN damage: loss of circadian coordination.
What are clock genes?
Per (Period) and Cry (Cryptochrome) make proteins that inhibit their own expression in feedback loop. Bmal1 and Clock activate Per and Cry expression. ~24-hour cycle results from delays in transcription, translation, protein degradation. Jeffrey Hall, Michael Rosbash, Michael Young won 2017 Nobel for discovering these in fruit flies (later confirmed in mammals).
What's melatonin?
Hormone produced by pineal gland. Released at night; suppressed by light. Promotes sleep. Used: jet lag treatment, sleep aid. Levels peak around 2-4 AM. Disrupted by: night shift work, evening screen use (blue light suppresses melatonin), aging (decreases). Effective sleep aid for some; not sedative — adjusts circadian timing.
How does jet lag happen?
Body clock out of sync with environment after rapid time zone travel. SCN adjusts ~1-2 hours per day. Crossing 6+ time zones: takes a week to fully adjust. Symptoms: fatigue, GI upset, cognitive impairment. Eastward worse than westward (shortening day vs lengthening). Treatment: light exposure at right times; melatonin; staying awake during destination day.
What's shift work disorder?
Health problems from working against biological clock. Increased risks: cardiovascular disease, diabetes, certain cancers, depression. Mechanism: chronic circadian misalignment; metabolism affected; immune function. Affects ~20% of workforce. Mitigation strategies: bright light at start of shift, dark periods, careful timing of meals/activity. Difficult: persistent tension between schedule and biology.
How are biological clocks studied?
Various methods. (1) Behavioral: activity patterns over time. (2) Hormone measurements: melatonin, cortisol levels. (3) Body temperature rhythm. (4) Genetic: clock gene mutations. (5) Free-running experiments: organism in constant darkness; reveals endogenous period (~24 h, slightly different per individual). Key insight: rhythms are internal, not just response to environment.