Like all animals, household pets like cats and dogs have circadian rhythms. These circadian rhythms affect a number of physical processes, such as when cats' eyes produce the most tears. However, circadian rhythms appear differently in pets because their sleep-wake cycles vary from ours.
For example, cats have an internal clock regulating their circadian rhythms just as humans do. It controls their sleep cycles, body temperature, and activity levels. However, unlike humans, cats have multiple short sleep cycles, rather than one full night of sleep.
Like humans, dogs do most of their sleeping at night. However, they tend to sleep in shorter bouts, and frequently nap during the daytime.