Hello, my name is***** and I have over 20 years of experience as a veterinarian. I'm sorry to hear that Gizmo is meowing half the night but she's certainly gotten your attention hasn't she?
Cats that are suddenly abnormally vocal are trying to tell us something, but unfortunately there are many problems that can cause yelling so it takes some detective work to figure out exactly what the problem (or problems) is (are). She is comforted by being in bed with you, so the problem may be in its early stages, but I do recommend looking.
Ideally she would have a physical examination but even if her physical examination is within normal limits, further diagnostic tests to find the cause may be needed.
I would start with a complete blood count, biochemistry profile and T-4 and a urinalysis.
Hyperthyroidism or an overactive thyroid gland is usually caused by a thyroid gland tumor. It can put cats on edge as they are overstimulated and can make them hungrier as well as it causes a ramped up metabolism where they burn more calories. Both of these things can lead to yowling. This can be diagnosed with a blood test.
A sore tooth, gum infections or a mass on the tongue or tonsils can lead to mouth pain and could be the cause of her yelling. She may be distracted during the day and only really become aware at night when all is quiet. She may need sedation by her veterinarian to fully examine her teeth, tonsils and tongue and diagnose these conditions but it won't hurt for you to take a look if she will let you.
Early senility or dementia often causes cats to vocalize more. They are literally yelling because they are lost and confused and are calling for help. This is usually a diagnosis of exclusion meaning that physically everything looks and tests out normal but they are still vocalizing.
Her sight may not be what it once was so leaving on night-lights to help her see may help and feel less anxious.
As animals age they sleep less soundly. It may help to play with her more during the day/evening so she is tired at bedtime and sleeps better. In some cases using music or "white noise" machines to block outside noise is helpful.
If her days and nights seem a bit mixed up it may help to give her a supplement called Melatonin. This is a naturally found hormone in animals and people that helps regulate the sleep/wake cycle and is involved in seasonal shedding. It can help them relax and sleep, and in cases that have abnormal shedding patterns related to seasonal light changes or abnormal growth hormone fluctuations. The usual dose in cats is 2mg to 4mg per cat every 12 to 24 hours. Make sure to give a dose 2 hours before bedtime.
Make sure to read the label and DO NOT use the fast dissolve tablets of Melatonin with xylitol as xylitol is toxic for cats.
We can use calming sprays or diffusors containing Feliway, a homeopathic drop added to her food or water called Rescue Remedy, or a supplement called Zylkene.
Drugs such as Anipryl (selegilene) can help with brain function. If that isn't enough then your veterinarian may prescribe a low dose calming medication like alprazolam.
In short it sounds like your girl needs a physical examination and minimally some blood tests taken to look for reasons for her "talking". If there isn't a physical cause then we can proceed with altering her environment and using things to reduce anxiety.
Please let me know if you have any questions.