A cat’s sleep cycle follows the circadian rhythm of its 24-hour biological clock. Like big-cat predators and their wildcat ancestors, a domestic cat’s clock instinctively works around the times when the cat would most likely hunt successfully, such as in the very early morning or at dusk. This means that most of their daylight hours are spent snoozing or in a deep sleep. Cats have a third eyelid, known as the translucent nictitating membrane, and when a shadow crosses it, the cat instantly becomes alert and ready to spring into action.
Most cats are able to adapt to their owners’ sleep/awake cycle. Well, sort of ...