Imagine this: Your character's light plane crashes in the middle of the Black Forest. You climb out of the wreckage in the evening. You run through the rapidly growing darkness, towards a mountain. You manage to climb partway up the mountain just in time to reach a clearing in the last light of day, and see a massive, dark forest spread out in front of you, with no sign of civilization.
At that point, the game would become non-linear, and you would be able to explore the forest at day or night. You would have a few objectives, find food, shelter, etc. If you went out during the day, everything would be normal, forest creatures, etc. But you would also find strange remnants from the night before, twisted, unidentifiable bodies, strange marks on trees, on the ground. At night, it would be very dark and foggy, and there would be a lot more ambient noise than during the day. You might get attacked by...
and that's where my idea runs out, I can't think of things to get attacked by without making it into a cliche.