Light is the clock

Morning light sets it. Evening light only nudges it.

There is a clock in the hypothalamus roughly the size of a grain of rice, and it does not keep good time unaided. Left without daylight it drifts — for most people, on a cycle slightly longer than twenty-four hours. Light is what corrects it, every day.

The correction is not symmetrical. Light in the morning pulls the clock earlier and does most of the work; light at night pushes it later. Getting outdoors in the first hour or two after waking is worth considerably more than anything done about screens at night, because outdoor light is ten to a hundred times brighter than a lit room and brightness is what the system actually counts.

An overcast morning outdoors beats a brightly lit room by an order of magnitude.

Evening light matters too, but the useful version of that advice is duller than the version that gets passed around: dim the room, not just the phone. A phone at arm's length is a small light. The ceiling fixture in a kitchen is a large one.

Consistency does more than intensity. A wake time that moves three hours at the weekend moves the clock with it, and Monday morning then arrives at a body still running on Saturday.

The longer answer

Screens before bed weighs the evening side: in the best-known laboratory study, four hours of reading on a tablet made people take about 10 minutes longer to fall asleep and pushed their body clocks later. A bedtime routine for adults puts daylight early and a dim house late into one routine, and tired but can’t sleep describes what a clock running late feels like at bedtime.