Oh gosh. I could never, ever do this. My head is GIGANTIC (I have a 25cm circumfrence, average female is 21-23) and if it falls lower than my shoulders I will throw up, and I have passed out before.
Best example was when I had to have a biopsy on my thyroid. They had to tilt the chair back and then have my head even farther so my glands were exposed. About halfway through, I couldn't take it any more. I blacked out, then woke up and promptly vomited all over the doctor and his lovely nurse.
Needless to say, I sleep propped up on a lot of pillows.