Melatonin as the body's calendar: light, seasons and mood
October 21, 2026 · 21 min read
Andrew Huberman's light episodes describe melatonin as the body's seasonal calendar, his episode on time perception calls circannual entrainment a calendar written in melatonin, his office-hours answers link seasons, melatonin, serotonin and mood, and Samer Hattar takes up night waking, melatonin and seasons with him. Seeing melatonin as a signal of the time of year, not just a sleep switch, helps explain why mood shifts with the seasons.
The rules, numbers and steps worth keeping in front of you.
- Light shuts melatonin off, so its nightly duration tracks night length and tells every cell the season.
- Huberman: any light that is bright enough shuts down melatonin; a bright overhead light at night can drop it to near zero.
- The direction of change matters: lengthening days signal falling melatonin, shortening days rising melatonin.
- Melatonin suppresses testosterone and estrogen; long days lift that brake.
- Huberman: the benefit is in the rhythm, not the amount, so be cautious with fixed, high-dose melatonin pills.
- Pregnant? Talk to a doctor before any melatonin.
- Avoid bright light between about 10 p.m. and 4 a.m.; from about 8 p.m. if prone to low mood.
- Huberman: 10 to 30 minutes of bright light within an hour of waking.
- Hattar: 10 to 15 minutes outdoors when sunny, 30 to 45 when overcast, up to an hour on very dark days.
- Sunlight through a window takes 50 to 100 times longer to set the clock.
- Hattar: find the minimum evening light you need; most people need about ten times less than they use.
- Hattar prefers warm, dim light to blue-blocking glasses; very dim red light under 10 lux doesn't disrupt sleep.
- Light at the wrong time can lower mood through a separate pathway; in Hattar's studies effects built over about two weeks.
- Winter lows: Huberman uses a SAD lamp or a 930 to 1,000 lux panel; see a doctor for seasonal depression.
This is the short form. The full insight has every expert's reasoning, their own words and the sources.