WASHINGTON (AP) -- 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.

Cohen wondered how both acute and chronic sleep loss interact with the body's natural circadian rhythms, the 24-hour biological clock that signals when it is time to sleep and to wake.

He recruited nine young, healthy volunteers and messed up their normally good sleep habits for three weeks. They stayed awake for 33-hour stretches with 10 hours of sleep in between, a radical enough schedule that their internal circadian clocks could not adjust. Their sleep deprivation was comparable to that of someone who gets about 5 1/2 hours of sleep a night, Cohen said, but the extra-long wake-sleep schedule also allowed him to test the value of catch-up sleep.

The volunteers' reaction times were tested every few waking hours, and compared to similar volunteers getting a normal amount of sleep.

The well-rested can catch up from the occasional all-nighter fairly easily. But as the study wore on and the volunteers became more sleep-deprived, the rejuvenation they felt each time they awoke increasingly proved a facade, Cohen reported Wednesday in the journal Science Translational Medicine.

They functioned OK during their first few waking hours, especially that first week. But then their reaction times steadily worsened with each hour they stayed awake, with a big drop in performance between the first and second weeks of sleep deprivation, he found.

That daytime decline was subtle, and the people's circadian rhythms provided a bit of rescue. Know how most people get a bit tired in the afternoon? Even these sleep-deprived volunteers got an energy boost then, as their circadian rhythms kicked in.

But when they stayed up past bedtime yet again, their performance suddenly plummeted just as their circadian rhythm reached its natural lowest point, Cohen's team found. The drop was so sharp that he concluded these people were increasingly vulnerable to accidents and errors. Although measured in milliseconds, the change was enough to mean a difference between, say, hitting the brakes in time or not.

"When exposed to the next all-nighter, they really fall apart much faster than they previously would," said Cohen, also a neurologist at Harvard Medical School's Beth Israel Deaconess Medical Center.

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