Why does cat meow in her sleep?

Why does my senior cat meow in her sleep?She does this once in a while and usually only once a night.

She's dreaming & the dream is so vivid that her mind is overcoming the natural paralysis that any dreaming predator animal (and human) has during the REM sleep stage. In her mind, she's making a lot more noise.
   
All the leg-twitching is also a dream, probably chasing something or being chased.
   
Cats & dogs sort out their daily experiences the same way we do, subconsciously & with dreaming.
   
All predators experience REM sleep & dream; prey animals like rabbits and rats don't, as REM sleep is so deep, it would make them too vulnerable to attack - so nature compensates by making sure bunnies are alwasy a little awake at all times.

Thanks for that!
   
I was worried that she might be in pain.
   
She has regular visits from a vet who comes to my house and she is in good health, but i just was startled by her meows since i usually am sleeping when she does it!