रात्रि प्रत्येक 24 घण्टों के दिन के दौरान सूर्यास्त से सूर्योदय तक परिवेशी अन्धकार की अवधि है, जब सूर्य क्षितिज के नीचे होता है। सही समय जब रात्रि शुरू होती है और समाप्त होती है,स्थान पर निर्भर करती है और ऋतु और अक्षांश जैसे कारकों के आधार पर पूरे वर्ष बदलती रहती है।

Night or nighttime is the period of darkness when the Sun is below the horizon. The opposite of nighttime is daytime. Sunlight illuminates one side of the Earth, leaving the other in darkness. Earth's rotation causes the appearance of sunrise and sunset. Moonlight, airglow, starlight, and light pollution dimly illuminate night. The duration of day, night, and twilight varies depending on the time of year and the latitude. Night on other celestial bodies is affected by their rotation and orbital periods. The planets Mercury and Venus have much longer nights than Earth. On Venus, night lasts 120 Earth days. The Moon's rotation is tidally locked, rotating so that the near side of the Moon always faces Earth. Nightfall across the near the side of the Moon results in the lunar phases visible from Earth.