The guide

When does your body start making melatonin?

By GyroscopeUpdated November 14, 20266 sources

Your own melatonin usually starts about 2.5 hours before you go to bed. The best way to estimate it at home is the time you wake on a day with no alarm. In a 2005 test that estimate landed within an hour and a half of the measured start for 22 of 23 people. Wake at 8am on free days, and yours probably starts around 9:35pm.

That time is the anchor for everything else. The window to move your clock earlier is counted back from it, and so is the time light starts working against you in the evening.

Doesn’t everyone’s melatonin start at the same time?

It’s easy to think so. Plenty of articles say melatonin rises “around 9pm”, and a 1994 MIT paper did put the body’s rise at about 9pm. That’s a fair average for people on a typical schedule.

But your clock isn’t average. A person who wakes at 6am on free days and one who sleeps until 11:30am are living about four hours apart inside, even if they share an office. If both take melatonin at the same clock time, it lands at a different point in each one’s night.

People also guess from the wrong morning. Your alarm tells you when work needs you. A morning with no alarm tells you when your body was done sleeping, and that’s what your melatonin follows.

A simple way to picture it

Everyone carries a body clock set to its own time zone. A free morning, with no alarm, is the clock on your wall in that time zone. Your alarm is the time zone your job lives in.

Melatonin runs on the first one, which is why the window tool asks for your free-day wake time.

How do you estimate yours?

The estimate comes from one study at Rush University in Chicago. Helen Burgess and Charmane Eastman looked at 120 young, healthy sleepers who slept at home for 6 days. Half kept fixed bedtimes. The other half slept whenever they liked, in July, August and September, and most of them were students. Wrist monitors checked their sleep logs. Then each one spent an evening in dim light, giving a saliva sample every half hour, so the researchers could see the moment their melatonin started rising.

In the free sleepers, wake time tracked that moment closely, with a correlation of 0.70 on a scale where 1 is a perfect match. In the fixed sleepers it was 0.44. The free sleepers gave this rule, in hours on a 24 hour clock:

Your melatonin start = 0.80 x your free-day wake time - 8.83

For an 8am riser that’s 0.80 x 8 = 6.4, minus 8.83 is -2.43, and 24 - 2.43 is 21.57, which is about 9:35pm. The researchers then tried the rule on 23 free sleepers from an earlier study. It came within an hour of the measured time for 17 of them and within an hour and a half for 22.

The bedtime column uses the other figure from the same paper. Earlier studies of young adults found melatonin starts about 2.5 hours before bedtime and about 13.5 hours after waking. Eastman and Burgess used the same 2.5 hour gap in their 2009 travel guide, where the best time for 0.5 mg is 2 hours before your melatonin starts, or 4.5 hours before sleep.

If your usual bedtime is far from the one in the table, the estimate is looser for you. The window tool says so when your bedtime puts your start more than an hour and a half away from 2.5 hours before bed.

Where does the estimate stop working?

The free sleepers were young, mostly students, slept when they liked and were tested in summer. The authors wrote that the rule may not readily apply to older people, people with body clock disorders, travelers with jet lag or night workers. They suggested that people whose work sets their wake time could use weekend wake times instead.

The only way to know for sure is to measure it the way the study did, with saliva samples every half hour in dim light. Sleep clinics run that test. In the 2018 Australian trial of 0.5 mg for late sleepers, nobody got in unless their measured start came after, or within 30 minutes of, the bedtime they wanted. Their melatonin was starting at bedtime or later, which is why their nights felt wrong.

Why does an evening in bright light change it?

Your start time isn’t fixed. Light at your eyes holds melatonin back. In a 2011 study of 116 adults aged 18 to 30, people kept in ordinary room light in the 8 hours before bed had a later melatonin start in 99 percent of cases than when they were kept in dim light, and their melatonin night was about 90 minutes shorter.

Night after night, a bright evening drags your start later, and your free-day wake time follows it. That’s one reason the light plan comes with every window. Light does the same job, harder has the details.

What if I never get a free morning?