You may think that cutting sleep during the week and making up for it on the weekend is OK, but as an article in the current AARP The Magazine, explains:

That sleep deficit you’ve been accumulating has real and dangerous implications for your brain, and not just because it makes you sleepy during the day. Sleeping less than seven or eight hours a night has been linked to cognitive decline, memory loss and possibly even Alzheimer’s, new research shows.

There are many tasks the brain accomplishes while you sleep; shorten the hours the brain has to do them and they don’t all get done.