The Signal Your Brain Is Waiting For

Every internal clock needs something to keep it accurate, or it would slowly drift. For the circadian rhythm, that something is mostly light — specifically, the brain uses light exposure as its main cue for what time of day it actually is.

Morning light carries the most weight. Bright light shortly after waking tells the brain 'the day has started,' which sets off a chain of timing effects — including, roughly a day later, when melatonin gets released that night. Skip that morning signal regularly, and the whole chain runs a little late.

This is why the next level goes deep on exactly one habit: getting real light early in the day. It's not the only input to the clock, but it's the loudest one, and it's the one most people are underusing without realizing it.

What does this lesson identify as the main signal the brain uses to set the circadian rhythm?

Bright light shortly after waking tells the brain that the ____ has started.

According to this lesson, light exposure has little to do with setting the body's internal clock.