One night of good sleep is not enough to stay alert and attentive, U.S researchers said on Wednesday.

Many nights of too little sleep when the body’s rhythm says it is time to snooze have cumulative detrimental effects on how a person performs and could be a safety risk, the researchers wrote in the journal Science Translational Medicine.

“Insufficient sleep over multiple sleep-wake cycles causes performance to deteriorate much faster for every additional hour we spend awake, particularly during the biological night,” lead author Dr. Daniel Cohen of Brigham and Women’s Hospital in Boston said in a telephone interview.

On average, a person needs about eight hours a night to preserve performance, said Cohen, a neurologist. Acute sleep loss is being awake for more than 24 hours in a row and chronic sleep loss is getting only about four to seven hours of sleep per night, he said.

Cohen’s team tracked nine healthy volunteers - five men and four women - to see what effect a combination of acute sleep loss, chronic sleep loss and biological sleep rhythm might have on their ability to function.

The researchers found that while most participants caught up on acute sleep loss with a single night of 10 hours sleep, those with chronic sleep loss showed deteriorating performance for each hour spent awake.

The volunteers were kept in a hospital for 38 days and lived on various sleep cycles. They were tested every four hours to measure alertness and attentiveness.

Cohen said researchers know that three days is not enough to recover from chronic sleep loss, but they still do not know how many days or weeks may be needed.

Cohen said people may not realize that they have a chronic sleep debt because it slowly builds over weeks.

“We can falsely feel like we’ve recovered quickly from chronic sleep loss because recent sleep makes us feel relatively restored early the next day,” he said.

“People may be largely unaware that they are chronically sleep-deprived,” he said. “It’s when they then stay up and try to pull an all-nighter that they are much more vulnerable to sudden sleepiness, inattentiveness and ... potentially accidents and errors.”

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* Chronic sleep loss builds over time

* Residual effects on performance

* Days, maybe weeks needed to recovery

By JoAnne Allen

WASHINGTON (Reuters)