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Sleeping in on Saturday after a few weeks of too little shuteye may feel refreshing, but it can give a false sense of security.

New research shows chronic sleep loss cannot be cured that easily. Scientists teased apart the effects of short- and long-term sleep loss and found that the chronically sleep-deprived may function normally soon after waking up, but experience steadily slower reaction times as the day wears on, even if they had tried to catch up the previous night.

It is work with important safety implications in an increasingly busy society, not just for shift-workers but for the roughly one in six Americans who regularly get six hours or less of sleep a night.

"We know that staying awake 24 hours in a row impairs performance to a level comparable to a blood-alcohol content beyond the legal limit to drive," said lead researcher Dr. Daniel Cohen of Boston's Brigham and Women's Hospital.

But when the chronically sleep-deprived pull an all-nighter, "the deterioration is increased tenfold," Cohen said.

The National Institutes of Health says adults need seven hours to nine hours of sleep for good health. Regularly getting too little increases the risk of health problems, including memory impairment and a weakened immune system. More immediately, too little sleep affects reaction times; sleepiness is to blame for car crashes and other accidents.

The new work shows how two different sleep drives impact the brain, one during the normal waking hours and the other over days and weeks of sleep loss.

It has critically important ramifications for anyone who works "crazy hours" and thinks they are performing fine with a few hours of weeknight sleep, said Shelby Freedman Harris, behavioral sleep-medicine director at New York's Montefiore Medical Center, who was not involved with the new research.

"Don't think you can just bank up your sleep on the weekend, because it doesn't work that way," Harris warned.