One of two cats has just started this odd sleeping behavior recently.

From time to time, he would hop onto the foot of my bed when I went to sleep
and take a nap himself.  He would sleep there for a few hours and then go
run around (noisily) the house for what seemed like the rest of the night.

Now, he hops into my bed practically every night.  Doesn't bother me or my
fiance, of course, but the way he has started laying down has become quite
odd.  He sleeps on his back, with his front legs stretched as far forward as
he can get them and his rear legs stretched as far down as possible.  It
makes for one very long looking cat!

He is perfectly healthy and there doesn't appear to be any reason for his
new sleeping posture (other than, I guess, his preference for comfort).  He
purrs loudly while doing this too.  Quite funny looking to see a cat that
likes to stretch out like this!