If we're watching our weight and "blow it" on any given day, we're told it's okay because we can make up for it the next day and get back on track. But what about when we lose sleep? Can we catch up over the weekend and, say, sleep in on Sunday morning to make up for those late, late nights during the work week?

I get this question a lot. I think it's because people expect to hear something different from the truth: NO, you can't just pay off a sleep debt by sleeping late on the weekend. Sorry. The proof? Check out the following studies of late, which have changed some of the conventional thinking of many of us in the world of sleep science:

• In 2003, scientists at the Walter Reed Army Institute of Research examined the cognitive effects of a week of poor sleep, followed by three days of sleeping at least eight hours a night. The scientists found that the "recovery" sleep did not fully reverse declines in performance on a test of reaction times and other psychomotor tasks, especially for subjects who had been forced to sleep only three or five hours a night.
• In a similar study in 2008, scientists at the Karolinska Institute in Stockholm found that when subjects slept four hours a night over five days, and then "recovered" with eight hours a night over the following week, they still showed slight residual cognitive impairments a week later, even though they reported no sleepiness.
• Another study done this year at Walter Reed found that people recovered much more quickly from a week of poor sleep when it was preceded by a "banking" week that included nights with 10 hours of shuteye.