Like any real world there are daytimes and nighttimes, and it gets dark at night. If you find yourself unable to see anything all of a sudden, then it's worth checking the time('time'), and holding a lantern or torch if appropriate('hold lantern').

Beware, light sources do run out! So remove it('remove lantern') as soon as it becomes daytime; otherwise you'll end up in the situation that I used to have all the time when I first started, namely stuck in the middle of nowhere with a dead lantern, low hps, and some damn boar kicking my butt. This can get quite exciting.