Circadian Rhythms
The body runs on clocks. The master clock sits in the suprachiasmatic nucleus of the hypothalamus — about 20,000 neurons firing in synchronized 24-hour cycles. This drives rhythms in temperature, hormone release, alertness, hunger, cell division. Almost every tissue has its own peripheral clock, synchronized by the master.
The circa-dian (about a day) rhythm persists without external cues. People in caves without light or time cues settle into cycles of roughly 24.2 hours. The clock is internal, genetic, ancient — present in bacteria, plants, fungi, animals. It predates multicellularity.
Light synchronizes the clock to the environment. Photoreceptors in the eye (distinct from those for vision) detect blue light and signal the suprachiasmatic nucleus. Morning light advances the clock; evening light delays it. The clock adjusts daily to match sunrise and sunset, keeping the internal day aligned with the external one.
Disruption comes with costs. Shift workers have higher rates of obesity, diabetes, cardiovascular disease, cancer. Jet lag impairs cognitive performance for days. Chronic desynchronization (social jet lag, where weekend and weekday schedules diverge) correlates with metabolic dysfunction. The clock doesn’t forgive being ignored.
Alertness follows predictable patterns. Most people have peak cognitive performance in late morning, a dip in early afternoon (the post-lunch dip is real but not caused by lunch — it happens without eating), and a second peak in late afternoon. Individual variation exists: chronotypes range from extreme larks to extreme owls. But most people are somewhere in the middle, and most work schedules ignore this.
Sleep timing matters as much as sleep duration. Six hours at the right time may beat eight hours at the wrong time. Sleep medicine increasingly emphasizes not just how much but when. The restorative processes of sleep depend on alignment with circadian phase.
Light environment has changed radically. Humans evolved with fire as the only artificial light — dim, orange, flickering. Now we have bright, blue-enriched light from screens and LEDs, available 24 hours. This light signals “daytime” to the clock at moments when darkness would naturally signal “wind down.”
Blue-blocking glasses, reduced screen time before bed, bright light in the morning — these interventions aim to recreate something like natural light cycles. The degree of benefit varies, but the principle is sound: the clock evolved for natural light patterns, and artificial light confuses it.
Go Deeper
Books
- Why We Sleep by Matthew Walker — Neuroscientist on sleep’s role in health, learning, and creativity. Compelling but read critically (some claims overstated).
- The Body Clock Guide to Better Health by Michael Smolensky & Lynne Lamberg — Practical applications of chronobiology.
- Internal Time by Till Roenneberg — On chronotypes and social jet lag from a leading circadian researcher.
Essays
- The 2017 Nobel Prize in Physiology or Medicine — Awarded to Hall, Rosbash, and Young for discovering molecular mechanisms controlling circadian rhythm. The Nobel lecture is accessible.
Films
- Sleepless in America (National Geographic, 2014) — Documentary on sleep deprivation’s effects on health, safety, and cognition.