In Daytime, there is a large number of vehicles (especially in highways), Chumps, Gangsters and walking Police Officers mainly on a nearby sidewalk). If the player gets into trouble with the police, it is more likely to have officers chasing him. Vehicles' headlights remain off during daytime.
In nighttime, there is a minimal amount of vehicles, depending on what area they're in. (Urban areas have more cars during night) The same number goes for other NPCs, because they are most likely in their vehilces during nighttime. There is a higher chance of losing your wanted level in nighttime than in daytime, because there are less walking policemen, chumps and gangsters. Vehicles' headlights turn on during the night, and turn on automatically.