Circadian Rhythm

The circadian rhythm is an internal ~24-hour clock (centered in the brain's suprachiasmatic nucleus) that regulates sleep/wake timing, core body temperature, alertness, and hormone cycles including melatonin and cortisol. It interacts with sleep pressure (the buildup of the desire to sleep across waking hours) to determine when you feel alert or sleepy.

Key features for fitness and sleep:

• Light is the master cue. Morning daylight advances and strengthens the clock; bright/ blue light at night suppresses melatonin and delays sleep (see screen-time).
• The clock drives a daily rhythm in performance. Many measures of strength, power, and flexibility tend to peak in the late afternoon/early evening, roughly tracking the body-temperature peak. This is modest and trainable, not a hard rule.
• A natural post-lunch dip in alertness occurs in early-mid afternoon — a good window for short naps (see napping).
• Disruption costs. Shift work, jet lag, and large weekend sleep-ins ("social jet lag") desynchronize the clock, hurting sleep quality and recovery.

Supporting the rhythm: keep a consistent sleep/wake schedule, get morning light and daytime activity, and dim light in the evening (see sleep-hygiene).

• ~24-hour internal clock; light is the primary entraining cue.
• Strength/power often peak late afternoon/early evening.
• Post-lunch alertness dip in early-mid afternoon.
• Best supported by consistent timing + morning light + dim evenings.

• sleep-hygiene — practices that entrain the clock.
• sleep-architecture — the clock times the stages.
• screen-time — evening light disruption.
• napping — using the afternoon dip.