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.
Hi there, I should have added that she only howls at night time. It usually starts about 10pm, until I go to my bed, then I fall asleep and she gets vocal.
I do think I will need to take her to the vet and get a check-up as I can't continue with this, she has woken me, my husband and two younger kids, but not my 17yr old, he can sleep through anything