I have slept outdoors before (in real life), in a sleeping bag, and tho I didn't get a great nights rest, I wasn't tired when I woke up.
They just implemented it based on just the game assets.
Its like Bethesda doesn't know squat about outdoor survival or didn't bother to really look into it. The sleep method in survival mode is frustrating. Also you MUST sleep in a bed mattress, not a sleeping bag or even on the ground cause your character will still be tired which I found ridiculous. Not when I am scrounging around looking for a place to sleep just to save the game. I did feel like I was fighting for my life, but only during stressful gun fights and battles. I get it, its suppose to be this ultimate hardcore hard level of play. And when I finish said 'tasks' I don't feel any sense of accomplishment. Survival mode doesn't feel like feels like work.tasks. After playing in Survival Mode for quite awhile now I have come to the conclusion that it brings nothing fun to the game.