Cats love to sleep. They sleep in sunbeams. The sleep on couches. They sleep under couches. They sleep on window sills. Cats can sleep 18 to 20 hours per day, whether they are kittens, energetic adults or seniors at age 14. The reason is that cats have an instinctual need to save up their energy for ambush hunting those foil balls, pieces of string, balls of yarn and dozens of cat toys.
What about puppies and dogs? They have a bit more energy, but as it turns out they need their sleep, too.