The clock in your eyes
Your body runs on a 24-hour clock, and that clock is set by light. Not by willpower, not by routine, not by an app. Light reaching the eye in the early part of the day is the timing signal the whole day runs on: when alertness arrives, when it fades, when sleepiness starts building in the evening.
A clock that never receives a strong morning signal drifts, and a drifting clock sits quietly behind half the sleep complaints people type into search boxes at one in the morning.
Twenty minutes outside
The prescription is almost embarrassingly small: get outside within an hour of waking, for about twenty minutes. Not through a window. Outside, eyes open, no staring at the sun required or advised.
An overcast morning delivers several times the light of a brightly lit kitchen, and a clear one delivers many times more. The weather is not an excuse. It is barely a variable. A grey Tuesday in February still outperforms every lamp you own, switched on at once. Coffee in hand if you like. The point is the light, not the ritual.
Why indoor light is not enough
This is where most attempts quietly fail. People hear "light" and turn on every lamp in the house, then wonder why nothing changes. Domestic lighting, even a bright office, even a kitchen with all the spots blazing, sits far below the threshold the clock responds to.
Indoor light is built for seeing, not for timing. You can read perfectly well under it and still be, as far as your body clock is concerned, sitting in the gloom. The gap between bright enough to work in and bright enough to set a clock is not small. It is the whole story.
The evening half
The same system takes a second input, in the other direction. Dimming your surroundings from dusk onwards tells the clock the day is ending, which is the instruction it needs to begin the night. Bright overhead light at ten argues with that message.
You do not need candles and blackout blinds. Lamps instead of ceiling lights, screens turned down, the last hour of the day noticeably dimmer than the first. Morning light sets the start of the day. Evening darkness sets the end of it, and the two halves work on the same clock. A clock that gets both signals stops drifting, and the evenings start suggesting bed at a sensible hour on their own.
Twenty minutes of daylight in the morning, a dimmer house after dark. Nothing to buy, nothing to track. The clock does the rest, quietly, within a week or two.