She's not deaf? Or going deaf? We have two with deaf cat howl. Rescue Remedy is having a bit of effect on the worst one. I put three drops on the back of her head at bedtime. The worst one is also suffering dementia. Blood panel for her shows no thyroid or any problem other than early stage kidney failure.
Another idea, is she responding to early, very early, signs of morning breaking around your house? A neighbour on shift work comes or goes at that time and disturbs her? Could you try to somehow make her night time accommodations sound and incoming light proof?