I've been googling for 30 + minutes trying to find a good source this was all I found http://www.nytimes.com/1995/06/13/science/sleep-inducing-substance-found-in-feline-brains.html

I heard this "cat fact" on a tv program about cats, and other felines, a few years ago I think it was on animal planet or discovery channel. Unfortunately I usually only remember tidbits of info like this and I have no idea what show it was or much else about it.

They are sleepy because they are bored. Give your cat a toy or allow it to go outside. Cats in the wild only sleep 8-12 hours per day.

Fun fact, cats can activate their sleep hormone consciously. Say Snowball got bored and have nothing to do, he just wills himself to sleep.