Time of Day

The clock starts at 6:00. It is day from 6:00 to 18:00, and night from 18:00 to 6:00. Each complete cycle takes 12 minutes (6 minutes day and 6 minutes night). The only ways to interrupt this cycle are with Night Stalker's ultimate, Darkness, and Luna's ultimate, Eclipse.

Vision
During the night most heroes have restricted vision, some notable exceptions being Night Stalker with 1200/1800, Meepo with 1800/1800, Slark with 1800/1800 and Enchantress with 1800/1800. Most other heroes in the game have a day vision of 1800, and a night vision of 800 or at least a night vision below 1800.

Neutral Creeps
Note: this mechanic has not been implemented in Dota 2

During night time, most, if not all, neutral creeps go to sleep. When neutral creeps are asleep, they will need to be attacked first and have vision of a unit for them to be pulled from their location.