Three times. Within an hour of waking, for 10 to 30 minutes, to set your clock. Late morning to early afternoon, when the sun is above 30 degrees, for 10 to 40 minutes if you want vitamin D. Late afternoon, 10 minutes, to mark the end of the light day. Everything else is refinement.

There are three outings because sunlight is not one thing. Its spectrum changes with the sun's height. Just after sunrise the light is bright but carries almost no ultraviolet; this is what the clock in your brain reads. UVA arrives when the sun passes roughly 10 degrees above the horizon, UVB near 30 degrees, and the sequence runs in reverse through the afternoon. Each outing catches a different part of it.

"Just tell me when to go outside"

That line comes from app reviews, and the complaint is fair. Circadian advice tends to arrive as a rulebook: lux thresholds, wavelength charts, a dozen habits. One reviewer of a competing app put it plainly: "It's all or nothing which is annoying so it tends to be nothing." The schedule below is three appointments, not a lifestyle.

Outing one: within an hour of waking

The suprachiasmatic nucleus, the master clock behind your eyes, runs on a cycle of about 24.2 hours and drifts unless something resets it daily. The reset is light entering the eye, read by melanopsin in the retinal ganglion cells, most sensitive near 480 nanometers. It responds to brightness. An overcast morning sky is around 10,000 lux, a lit office 300 to 500, and the clock can tell the difference even if you cannot.

Go out within the first hour after waking. Ten minutes under a clear sky, 20 to 30 under cloud, facing the bright part of the sky without sunglasses. Ordinary glasses and contacts are fine. The 20 minutes after sunrise carry the sharpest contrast, from scattered twilight to the direct disk, and are the strongest single input the clock receives all day, but the outing is anchored to your waking time, not to the sunrise. Expect no vitamin D from it. That is not what it is for. In winter this is the outing that survives, and morning light in winter covers why it still matters when the sun cannot make anything else.

Outing two: the UVB window, if you want vitamin D

UVB reaches the ground in useful amounts once the sun is more than about 30 degrees above the horizon. The ozone layer absorbs it aggressively, and at low sun the path through ozone is long. The field check: if your shadow is shorter than your height, the sun is above 45 degrees and UVB is present. Where it first arrives, roughly an hour after sunrise in summer at mid-latitudes, is the subject of when does UVA start in the morning; UVB follows one to two hours after that.

The minutes depend on skin. For midday summer sun with arms and legs bare, Fitzpatrick type I and II skin makes roughly 1,000 IU in 10 to 15 minutes, type III and IV in 25 to 40 minutes, and type V and VI in 45 to 60 minutes or more. Stop before the skin turns pink. Previtamin D3 production plateaus near one minimal erythemal dose, the exposure that just reddens skin, so time past that point buys more inflammation and no more vitamin D. Reddening itself is the skin's adaptive response to UV (Vind 2024), not a simple injury, but it is still the signal to stop. If you take a photosensitizing medication, a tetracycline or a thiazide diuretic for example, these minutes do not apply to you.

This is a dose to be timed, not maximized. Three or four outings a week in season does the work.

Outing three: late afternoon

When the sun drops back below 30 degrees, UVB is gone and the light is dominated by red, infrared and UVA. Ten minutes facing the western sky in the last hour or two before sunset gives the clock its second bookend: the day is ending. The value of that signal is the contrast it creates with what follows. Evening light suppresses melatonin at a median of about 25 lux in adults, and some people respond at under 10 lux (Phillips 2019), so the outing works best when the house is dim afterward. The artificial light problem covers the indoor half.

How the times move through the year

The clock times shift every day, which is why fixed rules fail. At 40 degrees north, roughly Denver, Madrid or Beijing:

  • In June the sun clears 10 degrees about an hour after sunrise and stays above 30 degrees for around nine hours, from mid-morning to late afternoon on clock time.
  • At the equinoxes the UVB window shrinks to about six and a half hours around solar noon.
  • In December the sun never reaches 30 degrees at 40 degrees north. Outing two closes for the winter. At 35 degrees north it opens for barely two hours around noon; at 50 degrees north UVA itself does not arrive until 100 minutes after sunrise.

Outing one never closes. Outing three only moves earlier.

What to do

  • Morning, within an hour of waking: 10 minutes clear, 20 to 30 overcast, no sunglasses.
  • Midday, when your shadow is shorter than you: 10 to 60 minutes by skin type, arms and legs out, stop before pink.
  • Late afternoon: 10 minutes facing the low sun, then dim the lights indoors.
  • From November to February above 37 degrees north, drop outing two and keep the other two.

One reminder per event beats a rulebook

The three times are simple. Knowing when they fall today, at your latitude, in this month, is the hard part, and a phone does it well. One notification per event, with one sentence on why that moment matters, replaces the rulebook. Sunlight is Life is free, computes your sun times on the device, and sends a morning sun reminder anchored to your local sunrise; with a free account it adds a notification at each event, UVA rise, the UVB window opening and closing, and sunset.

Get morning sun reminders for your location →

The through line

Every schedule here traces the sun's height. Low sun after waking sets the clock. High sun makes vitamin D. Low sun again in the afternoon closes the day. Once you see the three as one arc, you stop asking whether you get enough sunlight and start asking whether you were outside at the right moments. That question has a daily answer.