Wow, awesome article!

Some things I can rule out right away.

She was spayed at 6 months, so it's not a heat cycle or something like that. It's not hunger, because right before bed (when it's at its worse) is when I feed them and she always eats right away. Our normal food regime is to get food right at bedtime and they freefeed overnight.

Nutmeg will be 9 in June, so she's entering her senior kittycin years. However, this is something she has always done (vocalizing, I mean). I've just always lived in houses where I could lock her out of the bedroom. When I had that option, I just left her to the run of the house with Tilly and she could howl to her heart's content. I now live in a bachelor suite so I'm not even convinced this is a new behaviour, maybe it's just something I'm noticing now?

We just went to the vet for the annual check up in January and she was given a clean bill of health. However, I will watch for any changes, be they subtle or drastic, and of course I would take her in immediately if I had any questions or concerns.

Last night I was able to keep her awake from about 7pm until I went to bed at 11pm and she didn't howl at all - she just curled up on the bed next to me and slept the night through!

Thank you all so much for your suggestions and comments. I will for sure watch for these things and keep them in mind!