Horse falling while sleeping

[JUDYK89@aol.com]

Okay, I'm not sure this is even a problem, called my vet and he said not to 
worry about it.  My mare takes a nap every afternoon, but just recently has 
started falling in her sleep.  Sometimes her knees won't lock, and sometimes 
they do and she just slides backwards until she's a few inches from the 
ground and then she wakes up. She never completely falls down, but has come 
pretty close.  

I've had this mare for 2 years and have never seen her do this before until 
this week.  It's pretty scary, almost looks like she's drugged.  She's fine 
when you ride her, or walk her.  When she wakes herself up by almost falling, 
she kind of shakes it off and gets a drink, but then falls asleep again.

I know she hates to lay down, we have pretty hard ground, want to get 
shavings but the wind is so bad right now they'd all blow away.  She's a 
grey, so I know when she lays down even at night, and it's not very often.

Any ideas?

Thanks,  Judy