There is a "hub" during the sleep period... Usually it is around the time that the person is beginning to drift off to sleep and the mind is nice and fuzzy and beginning to wander.

It's when this hub is reached that things can begin to occur, or be seen - experienced.

But many people experience things at all other times as well. I've walked down the street in broad daylight and suddenly thought to myself "OMG, there's been a car crash!" only to find a friend has indeed been involved in a car accident that wasn't far from where I was at the time.