Get outdoor light within an hour of waking, keep the same wake time every day including weekends, and make the two hours before bed dim. Light is the lever; the other cues are secondary. Most people feel the shift in three days and settle in a week, moving about one hour per day.

The clock you are resetting is the suprachiasmatic nucleus (SCN), two small clusters of neurons above the optic chiasm. Its natural period is about 24.2 hours, so left alone it drifts later by roughly 12 minutes a day. It is corrected by light reaching melanopsin-containing retinal ganglion cells, which respond most strongly around 480 nanometers and project straight to the SCN. The rule that follows is the phase response curve: light in the hours after your body temperature minimum, which sits about two hours before you naturally wake, pulls the clock earlier. Light in the late evening pushes it later. That curve is the whole toolkit.

Morning outdoor light is the strongest cue

The melanopsin system counts photons, and the numbers are lopsided. A well-lit office is 300 to 500 lux. An overcast morning sky is about 10,000 lux, twenty times brighter, and a clear one 25,000 lux or more even with the sun barely above the horizon. Ten minutes outside delivers more clock signal than a morning at a desk beside a window.

The signal is strongest in the first hour after sunrise and stays useful through mid-morning. It fades by about two hours before solar noon; the sky is bright, but the clock has taken its reading. It does not need UVB, so it works in January at 55 degrees north as well as in June. What it needs is your retina: no sunglasses, glasses and contacts fine, face toward the brightest part of the sky, never at the sun. Ten minutes on a clear morning, twenty to thirty when it is overcast. Morning light in winter covers the short-day version.

Evening darkness is the other half

The clock reads the evening too, and it is far more sensitive than people expect. Phillips 2019 found half-maximal melatonin suppression at about 25 lux in the median person, roughly a dim living room, with a 50-fold spread between the most and least sensitive individuals. An overhead LED at 200 lux after 9 PM is a delay signal, and it cancels part of the advance you earned that morning.

From two hours before your target bedtime: lamps below eye level, warm bulbs of 2200 K or lower, screens dim and at arm's length or off. Bedroom under 1 lux, dark enough that you cannot see your hand. The artificial light problem has the bulb and glasses specifics.

A fixed wake time does what a bedtime cannot

You cannot force sleep onset. You can force the wake time, and the wake time decides when the light arrives. Sleeping until 10 on Saturday after waking at 6:30 all week moves your light exposure three and a half hours later, and the clock follows. Monday morning then feels like an eastward flight across two time zones. Wittmann 2006 named this social jet lag, and it is the most common reason a reset fails. Hold the wake time within 30 minutes, seven days a week, until the new schedule feels normal.

Meals and caffeine are secondary cues

The liver and gut carry their own clocks, and those entrain to feeding time rather than to light. Eating within two hours of waking and finishing the last meal three hours before bed lines the periphery up with the SCN; it does not move the SCN much on its own. Caffeine does move it. Burke 2015 measured a 40-minute melatonin delay from the equivalent of a double espresso three hours before bedtime. During a reset, keep caffeine before noon.

The seven-day protocol

Pick the wake time you want. If it is more than an hour earlier than your current one, move in 30 to 60 minute steps each day rather than jumping. The example below uses 6:30 AM.

  • 6:30 AM. Up, lights on, no snooze. Same time on all seven days.
  • By 7:30 AM. Outside for 10 minutes clear, 20 to 30 overcast. Coffee on the porch, the dog, the walk to the train. No sunglasses.
  • By 8:30 AM. Breakfast. Last caffeine before noon.
  • 3:00 to 5:00 PM. A second 10-minute outdoor break, for alertness rather than the clock.
  • 8:30 PM, two hours before target bed. Overhead lights off, warm lamps only, screens dim or done.
  • 10:30 PM. Bed, room under 1 lux. If you are not asleep in 20 minutes, sit in the dark rather than reaching for the phone.

Days one and two feel like waking early on too little sleep. By day three the evening sleepiness arrives earlier on its own. By day seven the wake time no longer needs the alarm. Sunlight is Life can send a morning sun reminder timed to your local sunrise, which removes the one step people skip.

Set a morning sun reminder timed to your sunrise →

Travel and shift work, briefly

Flying east means advancing the clock: morning light at the destination, dark evenings, about a day per time zone. Flying west means delaying it: afternoon and early-evening light, no early-morning light for the first two days, and it goes faster. For two or three night shifts in a row, do not try to flip the clock; keep an anchor sleep of four to five hours at the same clock time every day, wear dark glasses on the drive home, and black out the bedroom. For permanent nights, treat the start of the shift as morning and give it bright light.

What does not work

  • Sleeping in to catch up. It delays the clock by exactly the amount you slept in.
  • Melatonin on its own. Timed low doses shift the clock a little; bedtime doses mostly sedate. Without morning light the shift does not hold.
  • A light box at the wrong time. 10,000 lux at 9 PM delays you; the same box at 7 AM advances you.
  • Going to bed early without waking early. The clock follows the wake time and the light, not the bedtime.

The through line

A circadian reset is not a trick or a supplement stack. It is the same signal your ancestors received every day without trying: bright sky in the first hour, darkness after sunset, food and activity in between. Give it the two cues it was built to read and it corrects itself in about a week.