How to reset your circadian rhythm

A small region of the brain called the suprachiasmatic nucleus keeps time for the rest of the body. It sets daily rhythms in alertness, body temperature and hormones such as melatonin and cortisol.

Alongside the clock, sleep pressure builds the longer you are awake, from a chemical called adenosine. You feel sleepy when sleep pressure is high and the clock says it is night. When the two line up, falling asleep is easier and you tend to wake up feeling more rested.

The clock does not keep perfect time on its own. It resets every day based on signals, and light is by far the strongest one.

Light in your eyes soon after you wake tells the clock the day has started. Morning light shifts your rhythm earlier, which over a few days makes it easier to fall asleep at night and wake in the morning.

Outdoor light is far brighter than indoor light, even on a cloudy day. A walk, coffee on the porch or a commute on foot within the first hour or so of waking counts. Looking through a window helps less, and you never need to look at the sun.

Daylight later in the morning and around midday helps too.

A consistent wake time anchors the whole rhythm, often more than a fixed bedtime does. If you sleep in two or three hours on weekends, your clock drifts later, and Sunday night and Monday morning get harder.

Try to keep your wake time within about an hour on weekends. If you went to bed late, a short nap the next afternoon disturbs the clock less than a long lie-in.

Caffeine blocks adenosine, so it hides sleep pressure without removing it. Its half-life is roughly five hours on average, which means about half of an afternoon coffee may still be in your system at dinner. That number varies a lot from person to person with genetics, pregnancy, some medications and liver function.

Late caffeine can make sleep lighter and more broken even if you fall asleep easily. Many people do better keeping caffeine to the morning and early afternoon. A common rule of thumb is to stop about eight hours before bed.

Meal timing is a weaker cue than light, but it still affects the clocks in your liver and gut. Eating at roughly the same times each day, with more of your food earlier, fits your daytime metabolism.

A large meal close to bed can keep you up with reflux or a busy stomach. Many people sleep better if they finish eating two to three hours before lights out.

Bright evening light, especially cool white light, pushes the clock later and delays the rise of melatonin. Overhead lights and screens held close to your face both count.

In the last hour or two before bed, switch to lamps with warm bulbs and turn the brightness down on your phone. Keeping the phone out of the bedroom also helps, since scrolling keeps you alert too. A dark, cool bedroom supports sleep once you are in it.

A short nap of about 20 minutes in the early afternoon can help with the normal post-lunch dip without hurting night sleep. Long naps or naps late in the day use up sleep pressure you need at bedtime. If you have trouble falling asleep at night, try cutting naps first.

Regular exercise at any time of day is linked to better sleep. Morning or daytime exercise outside gets you light at the same time. Some people find hard training late in the evening keeps them wired, so if that is you, move it earlier.

Night and rotating shifts put your schedule against the light outside, and standard advice does not fit well. People on shifts often do better with a regular sleep window on work days, blackout curtains, bright light during the shift and sunglasses on the way home. Shift schedules are worth planning with a clinician, especially if you are drowsy driving home.

Habits help many people, but some sleep problems need a medical evaluation. See a clinician if trouble falling or staying asleep has lasted months, if you snore loudly or someone has seen you stop breathing or gasp in your sleep, or if you are sleepy enough to nod off while driving or at work.

Other reasons to get checked include an uncomfortable urge to move your legs at night, acting out dreams, a sleep pattern that is very late and will not shift, and low mood or anxiety that is getting in the way of sleep. Sleep apnea, restless legs and insomnia are common and treatable. If you ever feel unsafe, call or text 988 in the United States.

Fitting all of this around a real job, commute or kids is the hard part. Calibrate, a free tool at calibrate.day, asks about your day and suggests what to change first, whether that is morning light, caffeine, screens or bedtime. You do it on your phone with no wearable. It is wellness education and does not diagnose or treat anything.