Overview

You have a clock in your brain. It’s called the Suprachiasmatic Nucleus (SCN). It keeps time even if you live in a dark cave. It tells your body when to sleep, when to wake, when to eat, and when to repair DNA. Life evolved on a spinning planet with a 24-hour day/night cycle, so we internalized that rhythm.

Core Idea

The core idea is Entrainment. Your internal clock runs slightly longer than 24 hours (about 24.2 hours). Every morning, sunlight hits your eyes and resets (entrains) the clock to match the Earth’s rotation. Without light, you would drift out of sync.

Formal Definition

Physical, mental, and behavioral changes that follow a roughly 24-hour cycle, responding primarily to light and darkness in an organism’s environment.

Intuition

Think of an orchestra.

  • Conductor: The SCN in the brain.
  • Musicians: Every cell in your body (liver, heart, skin) has its own little clock. The conductor uses light to keep everyone playing the same beat. If you fly to Tokyo (Jet Lag), the conductor is in a new time zone, but the liver is still in New York. The music sounds terrible (you feel sick) until they sync up.

Examples

  • Melatonin: The “Vampire Hormone.” It only comes out at night. It tells your body it’s time to sleep. Blue light (from phones) suppresses it, tricking your brain into thinking it’s noon.
  • Cortisol: The “Wake Up Hormone.” It spikes in the morning to give you energy.
  • Heart Attacks: Most heart attacks happen in the morning because that’s when blood pressure rises sharply to wake you up.

Common Misconceptions

  • I can train myself to need less sleep: You can’t. You can get used to being tired, but the biological damage (cognitive decline) is the same.
  • Night Owls are lazy: Chronotypes (Larks vs. Owls) are genetic. In tribal times, it was safer to have some people awake at night to watch for lions.
  • Zeitgeber: “Time Giver.” Any external cue that resets the clock (Light is #1, but food and temperature also count).
  • Social Jetlag: Living against your clock (e.g., an Owl waking up at 6 AM for work). It causes health problems like obesity and depression.

Applications

  • Chronotherapy: Taking medicine at the right time of day. Chemo is more effective and less toxic if given when cancer cells are dividing (which follows a rhythm).
  • Lighting Design: Hospitals and offices are installing “Circadian Lighting” that changes color (blue in day, red at night) to improve health.

Criticism / Limitations

  • Modern Life: Electric light has broken our circadian rhythms. We live in “eternal twilight.” This is linked to the explosion of cancer and metabolic disease.

Further Reading

  • Foster, Russell. Life Time. 2022.
  • Walker, Matthew. Why We Sleep. 2017.