Results show that, in both men and women, performance decreased significantly after six nights of sleep restriction and did not improve after two nights of recovery sleep, suggesting that complete performance recovery may require more than just two nights of extended sleep. Sleep restriction also caused subjective and objective sleepiness to increase significantly; however, sleepiness did improve after recovery.

"After one workweek of mild sleep deprivation, two recovery nights were adequate in improving sleepiness but not performance," said principal investigator Dr. Alexandros N. Vgontzas, professor of psychiatry and endowed chair in sleep disorders medicine at the Penn State College of Medicine and director of the Sleep Research and Treatment Center at the Penn State Milton S. Hershey Medical Center in Hershey, Pa. "The usual practice of extending sleep during the weekend after a busy workweek associated with mild sleep loss is not adequate in reversing the cumulative effects on cognitive function resulting from this mild sleep deprivation."

The study also found significant gender differences. Compared with men, women were found to have less subjective sleepiness and less performance deterioration during sleep restriction, as well as greater improvements after recovery. These differences were associated with increased amounts of slow-wave sleep, or "deep sleep," in women at baseline.

"In women, but not in men, deep sleep appeared to have a protective effect," said Vgontzas. "Women with a higher amount of deep sleep can handle better the effects of one workweek of mild sleep deprivation, and their recovery is more complete after two nights of extended sleep."

According to the American Academy of Sleep Medicine, slow-wave sleep, or deep sleep, is one of the sleep stages that make up a sleep cycle. Each complete cycle lasts about 90 to 110 minutes. Most adults will go through four to six cycles in a full night of sleep. During slow-wave sleep, there is an increase in high-amplitude, slow-wave brain activity, known as delta waves.

The study involved 34 normal sleepers: 16 men and 18 women. They had a mean age of 24.5 years. Participants spent 13 consecutive nights in a sleep lab, sleeping eight hours per night for the first four nights as a baseline measurement. Then sleep was restricted to six hours per night for six nights, followed by three recovery nights of 10 hours of sleep.

Sleepiness was measured subjectively using the Stanford Sleepiness Scale and objectively using the Multiple Sleep Latency Test. Performance was measured using the Psychomotor Vigilance Task. Measurements were taken on the fourth day for baseline data, on day 10 after one week of sleep restriction, and on day 13 after two nights of recovery sleep.

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