Hard time sleeping?

I read an article (no link) but it stated that if you figure out when you need to wake up and subtract 7.5 hrs you will be able to deduce when you should go to sleep.
Thought that was pretty cool so I've been doing it for about a week..I truly am good with 6.5 to 7 hrs, which I thought was about right on. Give it a shot if your having a hard time sleeping, now I don't feel guilty staying up til about midnight every night!