Night begins at different moments depending on who’s defining it. Astronomers, lawmakers, drivers, and your own body all use different thresholds. The simplest answer: for most practical purposes, night starts at sunset and ends at sunrise. But the sky doesn’t go fully dark until well after sunset, your body starts preparing for night before the sun is gone, and various laws draw their own lines.
The Astronomical Definition
Astronomers don’t treat sunset as the start of night. After the sun dips below the horizon, its light still scatters through the atmosphere in a gradual fade called twilight. NOAA and the National Weather Service break this transition into three stages based on how far the sun sits below the horizon.
Civil twilight ends when the sun is 6 degrees below the horizon. At this point, the sky is noticeably dark, streetlights are on, and you need artificial light to read outdoors. Nautical twilight ends at 12 degrees below the horizon, when the sea horizon is no longer visible to sailors. Astronomical twilight ends at 18 degrees below, when the sky is completely dark and even faint stars and galaxies become visible. True astronomical night only exists once the sun passes that 18-degree mark.
How long this takes depends on your latitude and the time of year. Near the equator, the sun drops steeply and twilight lasts roughly 70 minutes total. At higher latitudes in summer, the sun can skim along just below the horizon for hours, and in places like northern Scandinavia or Alaska, astronomical night may never arrive at all during midsummer.
How Your Body Defines Night
Your brain doesn’t wait for a specific clock time. It tracks light through specialized cells in your retinas that are especially sensitive to blue wavelengths. These cells send signals along a direct nerve pathway to a tiny cluster of neurons that acts as your master biological clock. When light levels drop, this clock triggers the release of melatonin, the hormone that makes you feel sleepy and signals “night mode” to nearly every organ in your body.
The threshold is surprisingly low. Research protocols measuring melatonin onset require ambient light below 10 lux to avoid suppressing production. For context, 10 lux is dimmer than a single candle at close range. A typical living room at night runs 50 to 200 lux, bright enough to delay your body’s sense that night has started. In rodents and birds, melatonin suppression has been documented at levels as low as 0.028 to 0.3 lux, and even freshwater fish show reduced melatonin at just 0.01 lux.
Your body also builds up a chemical called adenosine throughout the day. The longer you stay awake, the more adenosine accumulates in your brain, creating the pressure to sleep. This sleep pressure interacts with your circadian clock: adenosine acts directly on the light-receiving area of the clock, reducing its sensitivity to light input. So the longer you’ve been awake, the less your internal clock responds to ambient light, nudging your biology toward night regardless of what’s happening outside.
Night in the Eyes of the Law
Legal definitions vary by context and jurisdiction, but most are tied to sunset and sunrise rather than a fixed hour.
- Driving laws: Most U.S. states require headlights from sunset to sunrise. Virginia’s statute is typical: headlights must be on from sunset to sunrise, and also whenever visibility drops below 500 feet due to rain, fog, or other conditions. Some states add a buffer, requiring headlights 30 minutes after sunset or 30 minutes before sunrise.
- Federal labor law: The U.S. Department of Commerce defines nightwork for federal employees as the hours between 6 p.m. and 6 a.m. local time, a fixed 12-hour window that doesn’t shift with the seasons.
- Maritime law: International collision regulations require vessels to display navigation lights from sunset to sunrise. During periods of reduced visibility, like fog, those lights must also be shown during the day.
The practical takeaway: if you’re wondering when to turn on your headlights or when a night-shift differential kicks in, the answer depends on which rule applies. Driving laws follow the sun. Pay rules follow the clock.
How Dark Does It Actually Get?
Outdoor light levels vary enormously depending on cloud cover, moon phase, and artificial light sources. On a clear night with a full moon, ground-level illumination can reach about 0.25 lux. On a moonless, overcast night, it drops to roughly 0.001 lux. That difference matters to wildlife: nocturnal bees only forage on moonlit nights after twilight, and owl monkeys significantly reduce their nighttime activity during new moons, compensating by staying active longer the next morning.
Even light levels below 0.1 lux, often labeled simply “dark” in research settings, are enough to alter animal behavior and physiology. This means what humans casually call “night” may still be biologically significant light for many species. A streetlight, porch light, or even a bright phone screen introduces far more light than any natural nighttime source.
Why the Answer Shifts With the Seasons
Because all astronomical definitions of night depend on the sun’s angle, the timing changes throughout the year. In December in New York City, civil twilight ends around 5:00 p.m. and true astronomical night arrives by about 5:30 p.m. In late June, civil twilight doesn’t end until after 8:30 p.m., and astronomical twilight lingers past 10:00 p.m. The length of night itself ranges from roughly 9 hours at the summer solstice to about 15 hours at the winter solstice at that latitude.
Move closer to the poles and the swings get more dramatic. At 60 degrees north (the latitude of Helsinki or Anchorage), summer nights may never reach full astronomical darkness. Move toward the equator and the variation nearly disappears: nights stay close to 12 hours year-round, and twilight is brief.
If you need to know when night starts tonight at your specific location, NOAA’s solar calculator gives precise times for sunset, each stage of twilight, and sunrise for any date and coordinates.