A short afternoon nap can boost a person's ability to perform memory-based tasks, but only if they've learned the task well beforehand, says a U.S. study.

Harvard Medical School researchers showed that subjects who took a 45-minute nap between memory tests were more likely to improve their scores the second time than those who remained awake.

The catch is that only those who scored well on the tests before their nap — that is, those who learned the tasks well — showed real improvement in their repeat performances. Those who did well on the initial test but were not allowed to sleep did not see the same improvement in the second round.

Those who did not do well in the first memory tests were not helped by taking a nap. Their re-test results were similar to those who likewise did poorly on the first tests and then stayed awake. 

The study, published Friday in the peer-reviewed journal SLEEP, followed 11 men and 22 women with an average age of 23.

The participants were trained in various memory tasks, including learning their way through a maze and pairing words. Shortly afterward, 16 subjects took a 45-minute afternoon nap while 17 remained awake in the lab. After the nap, all subjects remained in the lab until the retest about two hours later.

While it's long been understood that good sleep boosts performance for all kinds of tasks, the Harvard study shows you must have already achieved a certain mastery of the task for a nap to lead to better performance.

"These results suggest that there is a threshold acquisition level that has to be obtained for sleep to optimally process the memory," said Dr. Matthew Tucker, the study's author. "The importance of this finding is that sleep may not indiscriminately process all information we acquire during wakefulness, only the information we learn well."

Meanwhile in France, the health minister launched a study last year into whether workers should be allowed to sleep on the job.

"Why not a nap at work? It can't be a taboo subject," Health Minister Xavier Bertrand said at the time.

About one in three French people suffers from sleep problems and 56 per cent complain that a poor night's sleep has affected their job performance, according to the ministry.