Light sets the clock

Every cell in your body runs on a clock, and the clock is set by light. That finding is mostly the work of a Harvard researcher named Charles Czeisler, who through the 1980s showed that the human circadian rhythm, the roughly twenty-four-hour cycle that governs sleep, hormones, temperature and alertness, is reset each day by the light that reaches the eyes, and that the timing of that light is everything.

The practical rules are simple and most people break all of them. Daylight within an hour of waking: five to ten minutes outside, more on a cloudy day, no sunglasses, not through a window, which cuts the signal. That morning light tells the clock it's day and sets the time for tonight's sleepiness, about fourteen to sixteen hours later. Then dim light after dark: the brain reads bright light in the evening, especially the blue light of screens and overhead bulbs, as afternoon, and holds off the sleep hormone. Candles, lamps, warm bulbs, and screens off or dimmed in the last hour.

The shift worker, the frequent flyer and the teenager with a phone in bed are all fighting the same thing: a clock set wrong by light arriving at the wrong time. Czeisler's work made it possible to set it right on purpose.

It's a key for the third month because it's attention to something invisible. You don't feel the clock being set. You feel the result twelve hours later, as sleepiness that comes on time or doesn't. Morning light is the single cheapest, most powerful sleep intervention there is, and it costs nothing but stepping outside, which you've done every day for ninety-four days. Now do it earlier.