When you have a busy week at work or too much going on at home, sleep is sometimes the first thing to go. Or maybe you always sleep less than five hours a night. But chronic sleep deprivation—what you might consider “casual” sleep deprivation lasting just five days—can be as bad for you as not getting any sleep at all (pulling an “all-nighter”).

Researchers at the University of Wisconsin-Madison published a study in the online edition of the Proceedings of the National Academy of Sciences that found that five nights of restricted sleep--four hours a night--affect the brain in a way similar to that seen after acute total sleep deprivation. The study was performed with rats, but the findings correlate to humans.

"Even relatively mild sleep restriction for several nights can affect an individual's ability to perform cognitive tasks," says Dr. Chiara Cirelli, associate professor of psychiatry at the School of Medicine and Public Health, who led the research. "For instance, recent studies in humans have shown that 5 days with only 4 hours of sleep/night result in cumulative deficits in vigilance and cognition, and these deficits do not fully recover after one night of sleep, even if 10 hours in bed are allowed. Sleep restriction can also increase resistance to insulin, leading to a risk of diabetes."

Cirelli and her team kept rats awake 20 hours a day over five days while continuously recording the animals' brain waves with a sophisticated EEG as they were asleep and awake. The EEGs measured slow wave activity (SWA), the best marker of an individual's need to sleep. Sleep is regulated by homeostasis, so the longer we stay awake, the more slow wave activity we have when we do sleep. This produces an intense recovery sleep, a “sleep rebound.”

But when we restrict our sleep, say, to four hours a night when you’ve got a deadline looming, the slow wave activity lingers even while we’re awake, and more so in certain areas, impairing cognitive function. It’s not something you can “catch up” on later, either. You need an adequate amount of sleep every night.

Missing sleep takes a toll on your beauty as well. Your skin feels dry and looks dull, your eyes get tell-tale dark circles, and your whole face is puffy. While you can often cover these up for a night or two, if you engage in chronic sleep deprivation you run the risk of this becoming your permanent look--and it's not a good one.

"Instead of going to bed when they are tired, like they should, people watch TV and want to have an active social life," Cirelli says. "People count on catching up on their sleep on the weekends, but it may not be enough."

So decline a drinks invitation and shut off your TiVo--whatever is taking up your sleep time needs you in a well-rested state with full brain function as much as it needs your overtime.