Caffeine has no benefit after 3 nights of sleep restriction

After restricting sleep to five hours per day, caffeine use no longer improves alertness or performance after three nights, a new study has found.

The results showed that relative to placebo, caffeine significantly improved Psychomotor Vigilance Task (PVT) performance during the first two days, but not the last three days of sleep restriction.

"We were particularly surprised that the performance advantage conferred by two daily 200 mg doses of caffeine was lost after three nights of sleep restriction," said Tracy Jill Doty from Walter Reed Army Institute of Research in the US.

"These results are important, because caffeine is a stimulant widely used to counteract performance decline following periods of restricted sleep. The data from this study suggests that the same effective daily dose of caffeine is not sufficient to prevent performance decline over multiple days of restricted sleep," said Doty.

The study group consisted of 48 healthy individuals who participated in a double blind, placebo-controlled study. Sleep was restricted to five hours of time in bed for a total of five days.

Participants were administered either 200 milligrammes of caffeine or a placebo twice daily. A cognitive task battery was administered hourly during the wake periods and included a 10-minute PVT, Profile of Mood States (POMS), and the Stanford Sleepiness Scale (SSS), researchers said.

A modified Maintenance of Wakefulness Test (mMWT) was administered six times per day, they said.

Researchers found that after restricting sleep to 5 hours per night, caffeine use no longer improved alertness or performance after three nights.

The findings were published in the journal Sleep.