IIRC the numbness is caused by nerve compression, not blood-flow cutoff.
edit: Apparently it can be either way: http://www.wisegeek.org/what-are-the-most-common-causes-of-numbness-while-sleeping.htm
edit2: And another source claims it’s due to nerves, so I dunno. I do find the nerve explanation more plausible than the blood-flow one.
IIRC the numbness is caused by nerve compression, not blood-flow cutoff.
edit: Apparently it can be either way: http://www.wisegeek.org/what-are-the-most-common-causes-of-numbness-while-sleeping.htm
edit2: And another source claims it’s due to nerves, so I dunno. I do find the nerve explanation more plausible than the blood-flow one.