From Liquipedia Warcraft Wiki
The time of the day is very important in Warcraft 3 as day and night each causes certain advantages and disadvantages.
The Day/Night indicator on the top of the screen shows the current time of the day. During the day it is indicated by a sun, during night by a crescent moon. The eight circles show the progress of the current day/night. Once all eight circles are filled the time either changes to day or night. There will be a bluish glow for night, and a yellowish glow for daytime as well. When day turns to night it is also indicated by a wolf howl.
Buildings, units and heroes have a greatly reduced sight range during nighttime, which makes scouting and moving troops during nighttime harder.
Night Elves can research the Ultravision ability that gives them the ability to see as far at night as they do during the day.
While Orc and Human (non-Hero) units regenerate hit points both during daytime and during nighttime, Night Elf units only regenerate hit points during nighttime. Undead units regenerate hit points during day- and nighttime but only on blight.
Certain abilities only work during nighttime, like the Night Elfes' Shadowmeld ability. Also Moon Wells only regenerate mana during nighttime.
During nighttime, most creeps will fall asleep. During this period they will not attack, unless woken up. They will wake up when attacked, when a building is constructed too close or when a player uses a nearby Neutral Building.
Some creeps do not sleep, and will wake the rest of the creep camp if the player gets too close.
Day/Night Cycle Manipulation
Night Elves can buy the Moonstone item from their Ancient of Wonders which causes an eclipse that blocks out the sun and creates an artificial night for 30 seconds. During that period of time, the Day/Night Indicator has a purplish glow indicating that it is modified. Moonstones are very useful to refill Moon Wells, especially after researching Well Spring.