Cats do not have the same daily sleep-wake cycle that we and many other animals do. Rather, they sleep and wake frequently throughout the day and night. This is because cats in the wild need to hunt as many as 20 small prey each day; they must be able to rest between each hunt so they are ready to pounce quickly when prey approaches. This explains why our cats seem to sleep so much during the day when we are awake, and spend so much time awake at night when we need to sleep!
We may conclude that cats are "creatures of the night", but it only seems that way because their sleep/wake cycle is so different from ours. Adding playful activities to your cat's daily routine can diminish the occurrence of such nocturnal activity (see more information about increasing the activity of your cat).