First of all, the bottom line is : Don't go out at night. At night, great concentrations of doom-spawn appear and are driven by a furious rage to kill on sight. While at first you may be attacked by weakling Doom-minions, later you may be attacked by the thuggish Men of the South, or the merciless Valeriya.
Okay, so onward with the tutorial:
After going for a while, the game will slowly tint to sunset, and then to night. - Make sure you are inside when sunset starts waning. If you stay outside, you will be attacked, and probably die.
Other than simply waiting for the sun to come up, you can use beds spread out through the destroyed houses and bases. One most easy to find is the bed in the Dragon Inn.
I hope this explains a bit more about the importance of evading open spaces at night.
