How LightOtter works it out
LightOtter gives you a daily rule of thumb for daylight outdoors. This page explains how it is worked out and how well it is supported.
The rule of thumb
The base comes from cloud cover: about 10 minutes when it is clear, 15 when partly cloudy and 20 to 25 when overcast. With a low sun it goes up, to 30 minutes at most. It is an estimate, not a measurement, and not a prescription: research does not name one number of minutes that fits everyone.
The winter rule
On very short days with less than about 9.5 hours of daylight, and in the dark months (November to February, May to August in the southern hemisphere) when the sun rises more than an hour after your wake-up time, LightOtter simply uses about 25 minutes outside once it is light. That is why the minutes in the “Morning daylight by city” table jump to 25 in some months. In the app you can also switch winter mode on or off for good.
The light windows
Morning: from sunrise, starting a little later depending on your chronotype. Midday: around the sun’s highest point, short and with no reward for staying longer; with very high UV, LightOtter suggests shade. Evening: the last hour and a half before sunset, then darker.
Broad agreement
A rhythm of bright days and dark nights matters. Morning light can shift the body clock earlier, light late in the evening can shift it later. What counts is visible light reaching the eyes, not UV (Brown 2022, Spitschan 2025, Piper 2026).
Observed
In a large observational study, bright days and dark nights were statistically linked to a lower risk of death (Windred 2024). That is an association in observational data, not proof of cause and effect, and the study did not test LightOtter.
Open questions
Exactly how many minutes, for whom, and how much window light or clouds count is not settled. That is why LightOtter sticks to an honest rule of thumb and counts every day from 2 minutes outdoors.
What LightOtter is not
LightOtter is not medical advice and not a medical device. If you feel tired for a long time, talk to a doctor. Never look directly at the sun.