Recovery sleep 'can't fully fix some cognitive deficits'

Earlier this month, Duke-NUS Medical School researchers were featured in a first-of-its-kind study in the United States journal Sleep.

In the study, 56 teenagers, mostly from "elite" high schools and aged between 15 and 19, stayed in a boarding school for two weeks to assess how insufficient sleep could affect cognitive function.

For seven nights, half the teens received five hours of sleep, while the other half had nine hours - the recommended sleep duration for this age group.

Sleep duration was objectively monitored using an electroencephalogram and wrist-worn motion sensors.

Participants were assessed three times a day using a computerised battery of tests, in addition to other assessments. In between testing, they were allowed to interact, read, study and watch movies.

Each battery of tests lasted about 25 minutes and comprised tasks that evaluated sustained attention, speed of processing, executive function, subjective sleepiness and mood.

Researchers found that those in the nine-hour sleep group either maintained cognitive performance or showed practice-related gains in the speed of processing tasks that required arithmetic calculation or symbol decoding.

On the other hand, the sleep-restricted group, who slept only five hours a night, showed significant and cumulative deterioration of sustained attention, working memory and executive function.

A sobering discovery was that two nights of nine-hour recovery sleep could not fully reverse some of these cognitive deficits.

Positive mood and subjective alertness were also affected, but there was a significant dissociation between objective and subjective markers, said Professor Michael Chee, senior author and director of the Centre for Cognitive Neuroscience at Duke-NUS.

For example, in the sleep-restricted group, sustained attention continued to deteriorate across the seven days, while sleepiness declined until the third day and remained about the same until the end of the experiment, said Prof Chee.

This mirrors what happens in real life, where students do recognise that they are tired, but underestimate the extent to which their performance declines as a result of sleep deprivation.

Tan Weizhen

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A version of this article appeared in the print edition of The Straits Times on March 29, 2016, with the headline Recovery sleep 'can't fully fix some cognitive deficits'. Subscribe