Sleep & wake-up
Daylight hours calculator
Estimate the hours of daylight for a latitude and date, and how fast the day is getting longer or shorter.
Daylight length depends on latitude and date only, not longitude. The higher the latitude, the more the day stretches and shrinks across the year.
How daylight length is calculated
The length of a day depends on just two things: how far you are from the equator (your latitude) and the date, which sets the Sun's declination, the angle of the Sun north or south of the equator. This calculator uses the sunrise equation in the form popularised by Forsythe and colleagues (1995): it converts the day of the year into the Sun's declination, then works out the hour angle at which the Sun crosses the horizon for your latitude. The result is the time from sunrise to sunset. A small correction of 0.833 degrees is included so the day is measured to the moment the Sun's upper edge meets the horizon, matching published sunrise and sunset tables.
Longitude and time zone are deliberately left out, because they only move sunrise and sunset earlier or later on the clock; they do not change how long the day lasts. At the equator the day stays close to 12 hours all year. The further you go towards the poles, the more extreme the swing, until beyond the Arctic and Antarctic circles you reach full days of sunlight in summer and full nights in winter.
| Latitude | Longest day (June) | Shortest day (December) |
|---|---|---|
| 0° (equator) | ~12 h 7 min | ~12 h 7 min |
| 40° N | ~14 h 52 min | ~9 h 20 min |
| 51° N (London) | ~16 h 38 min | ~7 h 50 min |
| 66.5° N (Arctic circle) | ~24 h | ~0 h |
Daylight, light and your body clock
Near the equinoxes in March and September the day changes fastest, by two to three minutes each day at mid latitudes, which is the figure the tool reports for dates around now; near the June and December solstices the length barely moves for a couple of weeks. This matters for your body clock, which uses daylight as its main timing signal. Getting bright light soon after waking, especially in the darker months when mornings are short, helps anchor your sleep and wake times; bright light late in the evening pushes them later. If winter days feel heavy, more morning daylight is a reasonable first step, and persistent low mood through the dark months is worth raising with a doctor.
Frequently asked questions
Why does the calculator not ask for longitude or time zone?
The length of a day depends only on latitude and date. Longitude and time zone shift the clock times of sunrise and sunset but leave the total daylight the same, so they are not needed to find the number of hours.
How accurate is the sunrise-equation estimate?
It is usually within a few minutes of published tables for most locations. It ignores altitude, local terrain and unusual refraction, and it becomes approximate very near the poles, where the day flips between 24 and 0 hours.
Why do days change length fastest in spring and autumn?
Around the equinoxes the Sun's declination moves quickest, so sunrise and sunset shift two to three minutes a day at mid latitudes. Near the solstices the declination nearly stalls, so day length changes very little.