I haven't had any experience myself but my cousin's elderly cat did this for a while and I have also read of this behaviour in cat advice books.
It's apparently relatively common in elderly cats and could be a sign of 'dementia' insofar as the cat panics when it wakes at night, unsure of where it is and needs the reassurance of familiar and comforting people and attention. During daylight the cat acts fairly normally.
With my cousin's cat she started to bring her upstairs to her bedroom at night so that when she woke she could see the person she felt most secure with. However, you say that your cat is already on your bed when she starts yowling. Perhaps just say a few reassuring words and don't get up because this probably reinforces the behaviour.
It might be worth getting the vet to give her a check-up just to exclude any medical problems. I know you said your cat was healthy but I believed my 16 year old cat was very healthy too - she had her annual vet check-up but less than 6 months later she had died from cancer. The annual check is, after all, fairly cursory.