Why do cats sleep all the time?
In their wild or feral state, cats need to spend a good proportion of their time hunting in order to catch enough food. Because the cat hunts alone, he only has himself to rely on to find enough food to survive, so has to keep going until he is successful. Hunting tends to take place when the natural prey species of the cat (small rodents) are active, which is mostly late in the evening and early in the morning. Because hunting is hard work for cats, they will tend to rest and sleep for much of the rest of the day to conserve energy. It often seems to us that our cats sleep a lot because they sleep during the day when we are up!
There are also differences in sleeping habits between individual cats: some cats are very inactive and will rest for a large proportion of the day and night. This particularly occurs when their activity patterns are inhibited in some way: for example indoor cats that do not have an adequately enriched environment may become very inactive.