Basically, the cats body clock becomes the oddest it's ever been....and they let you know by screaming at silly o'clock in the morning! There's also obvious signs of being disorientated and easily confused.
This is a pretty good checklist:
1. Spatial disorientation or confusion - getting trapped in corners or forgetting the location of the litter box. House-soiling is the most common reason for referral of old cats to animal behaviouralists.
2. Altered relationships either with their owners or other pets in the household - increased attention seeking or aggression
3. Increased irritability or anxiety, or decreased response to stimuli
4. Changes in sleeping patterns
5. Inappropriate vocalisation such as loud crying at night
6. Loss of memory, such as forgetting commands or breaking house training
7. Changes in activity - aimless wandering or pacing, or reduced activity
8. Lack of interest in food, although some cats actually want more food.
9. Decreased grooming
10. Confusion about time, such as forgetting that they have just been fed
Old Mog has about eight of these symptoms.....
Bless her tired old socks!