My Mum's cat used to be like that - You would put her upside down inbetween your legs and she would fall asleep within 10 seconds with her head flopping over your knees!!! I suppose its just like when kids will fall asleep in any position!!
Its because they put all their trust in you and know that you won't let them fall.
She reminded me of when my boys were newborns and thier little necks weren't strong enough to hold thier heads up because if I would have let go, her head would have flopped back
Lily not be naughty? Boy does she have you all fooled!
this is the same kitty that has tore 2 shower curtains, knocks over glasses and waits till about 3 in the morning to do her 300 mph laps around the house including right across your head!