You should always ask yourself: "How could anyone possibly know this?"
There are so many factors that would be involved. For starters, how many spiders do you think are currently in the room where you sleep? Personally, for me, I would guess zero. I'm in the room right now, and there aren't many places to hide. Spiders are pretty obvious.
Here's another problem: you know how sometimes, you wake up because of a very slight disturbance? For example, once I woke up because a piece of paper had slipped off my desk and was touching my face. Well, if spiders occasionally get into your mouth and down your throat, don't you think that every once in a while, you would wake up while it was on your face or in your mouth?