Night Length Calculator
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Current Local Time
—This night length calculator shows the total hours of darkness between sunset and the next sunrise for any location and date.
What Is Night Length?
Night length measures the total hours of darkness between sunset and the following sunrise at a specific location. It is the mathematical complement of day length — when you subtract day length from 24 hours, you get night length. The same factors that shape day length — latitude, axial tilt, and orbital position — drive night length in the opposite direction.
Near the equator, night length remains close to 12 hours throughout the year. At higher latitudes, it can stretch to 18 hours or more in winter, and near the poles, entire weeks can pass in continuous darkness during polar night. This calculator computes the exact hours and minutes of darkness for any location and date, accounting for the fact that night is always slightly shorter than 24 hours minus day length — because atmospheric refraction extends daylight and shortens darkness.
How Our Night Length Calculator Works
Our night length calculator computes the duration between sunset and the next day's sunrise using the SunCalc library and Jean Meeus algorithms. It applies standard atmospheric refraction of 0.833° and runs entirely in your browser.
The tool calculates today's sunset time and tomorrow's sunrise time for your coordinates and date, then determines the difference. The algorithm accounts for atmospheric refraction, observer elevation, and geographic position to deliver night length in hours and minutes, accurate to within seconds of observational data.
FAQ — Night Length Calculator
How long is tonight where I am?
Use the calculator above and enter your city or coordinates to get tonight's exact night length. In my experience, the results are accurate to within seconds. If you're planning astronomy or astrophotography, remember that astronomical night (when the sun is more than 18° below the horizon) is shorter than the calculated night length — it doesn't begin until civil and nautical twilight end.
Is night length always the opposite of day length?
Almost — night length is 24 hours minus day length, but atmospheric refraction means night is always slightly shorter than you'd expect. Refraction extends visible daylight by a few minutes at both dawn and dusk, which compresses the night period. At most latitudes, this difference is small — typically 5-10 minutes — but it's measurable.
What happens to night length near the poles?
Near the Arctic and Antarctic Circles, night length can reach 24 hours during winter — the phenomenon called polar night. Conversely, in summer the opposite occurs: zero hours of night (midnight sun). The transition between these extremes is dramatic — at 70°N, night length swings from 24 hours in December to 0 hours in June.
How does night length affect wildlife?
Many nocturnal animals time their activity to night length — longer winter nights mean more foraging time for owls and bats, while shorter summer nights compress their activity windows. Migration patterns in birds are triggered partly by changing night length. I've noticed that in my area, the return of longer nights in autumn coincides with increased owl activity.
Can night length data help with sleep planning?
Yes — understanding the natural light-dark cycle helps optimize sleep hygiene. During short winter nights, you may need to supplement with light therapy to maintain alertness. During long summer nights, blackout curtains become essential. I use night length data to adjust my sleep schedule seasonally, aligning my wake time closer to sunrise.