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Your clock averages recent mornings
The clock averages days
Because it integrates photons across days, a rushed morning in sunglasses can be offset by a longer stretch outside the next day. A bright, clear morning followed by a dreary one is averaged in the same way. The clock decides when cortisol should rise and melatonin should come on according to the previous two or three days of early daylight.
Light and other timekeepers
Bright light after waking is the biggest timekeeper, or zeitgeber, in the circadian literature. Exercise, eating, and social interaction contribute too, and late-day light matters, which is why lights stay low late in the evening.
Jet lag and slack
The same slowness explains jet lag. After travel, the clock still runs on the previous city's schedule and takes a few days to shift. At home that lag leaves room to move: twenty minutes outside one day, five the next, and fifteen after that all fit within what the clock averages, with more photons better and fewer photons calling for a little more time.
How does light reset the master clock?
Light signals reach the suprachiasmatic nucleus through a pathway from the retina and trigger expression of clock genes such as Per1 and Per2. Those genes belong to a feedback loop of proteins that rises and falls over about 24 hours inside each clock cell. Shifting their activity shifts the phase of the whole loop. Jeffrey Hall, Michael Rosbash, and Michael Young won the 2017 Nobel Prize in Physiology or Medicine for uncovering this molecular machinery.
Why does eastward jet lag feel worse?
Charles Czeisler and colleagues reported in Science in 1999 that the average human body clock runs a little longer than 24 hours, about 24.2. A longer natural cycle makes it easier to delay the clock than to advance it. Flying west asks for a delay, while flying east asks for an advance. Travelers commonly adapt more slowly after eastward trips.
How did bunker studies reveal the body clock?
In the 1960s, Jürgen Aschoff in Germany housed volunteers in an underground bunker with no clocks or daylight. Their sleep and wake cycles continued on a roughly daily rhythm that was close to, but not exactly, 24 hours. Aschoff coined the term zeitgeber, German for time giver, for the outside cues that keep that rhythm aligned to the day.
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