State of Decay, uses a very common open world feature, a day and night cycle, day and night have their own advantages, and disadvantages. Daytime lasts for an hour of real time, followed by an hour of nighttime, which makes in total two hours for a 24 hour day cycle.
In daytime, everything is easier, as you can see better. It has allowed number of wandering zombies and they will have a harder time trying to sneak up on you.
Nighttime is the more difficult of the day, as it is darker, so zombies can easily sneak up on you, and also can get you disoriented. Zombies are more active at nighttime, so be wary when venturing at nighttime, by yourself, as it can be a death wish for beginner players.