Keep the circadian clock on time

Use stable timing and light so your internal clock and your sleep window agree.

Why it works

A master clock in the suprachiasmatic nucleus sets a ~24-hour rhythm for alertness, body temperature, and melatonin. It does not keep perfect time on its own; it is entrained mainly by light. When your light and timing cues drift, the clock and your desired sleep window fall out of phase, and you lie awake or wake too early even when tired.

How to do it

1. Get bright light (ideally outdoor) within an hour or two of waking.
2. Keep wake and sleep times within about an hour, including weekends.
3. Dim and reduce bright light in the last hour or two before bed.

Evidence

Light as the dominant entrainer of the human circadian clock, and the SCN as the master pacemaker, are well-supported in chronobiology; morning light advances and evening light delays the clock. (rct)

Individual chronotype and light sensitivity vary, so exact timing is personal even though the direction of effect is reliable.

Sources

• Czeisler et al. (1986), bright light resetting of the human circadian pacemaker, Science

Common mistake

Sleeping in for hours on weekends, which shifts the clock later ("social jet lag") and makes Monday morning feel like a time-zone change.