When I lay down to go to sleep lately, I will close my eyes or get very relaxed and when I open them - even if I'm not being startled - i'll get this feeling of light headedness that increases until I feel like I'm going to faint. Often I'll have to get out of bed and sit down on the ground (in the position your suppose to when you faint) to make it go away. Once it goes away, I'll get back into bed only to feel the same way when I begin to fall asleep again.
Interestingly, I'm thinking this may be caused by anxiety because if I nap during the day or wake up in the morning and fall back to sleep, I do not experience it. I do have a history of Anxiety.