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Cats are nocturnal in outdoor environments but usually revert to normal human sleep patterns when they are kept entirely indoors.  About the only change normally seen in sleep patterns in elderly cats is that they sleep many more hours a day. 

Elderly dogs, however, also sleep many more hours a day, they have a tendency to develop a reversal of their sleep patterns and tend to be up more at nights. This is particularly true when they start developing senility (cognitive dysfunction) disorders.  This can be bothersome to the owners as they tend to intermittently awaken the owners as if they need to go outside to urinate or defecate but they tend to just walk around and then want to return inside. 

Additionally, older adult and elderly dogs can develop various sleep disorders, usually thought by the owners as dreaming.  These disorders are not severe nor or they common, but usually result in awakening the owner multiple times during the night.   Rarely, younger adult dogs, especially the Doberman breed develop true REM (rapid eye movement) sleep disorders that can be very severe.   There have been cases of these dogs killing the owner while both the dog and owner are sleeping.