A midday nap can help pre-school children remember what they learned in the morning, according to a study by psychologists in the US.
The research suggests that carers and nurseries that phase out after-lunch sleeps may be harming children's ability to learn, by disrupting the way their brains store memories.
Pre-schoolers who went without a midday sleep fared worse on memory tests than those who napped. They also failed to improve their scores even after a good night's sleep, the researchers found.
The findings highlight the crucial role of sleep in consolidating memories, a process that underpins the brain's ability to learn new information.
The children who benefited most were "habitual nappers" who would sleep when carers encouraged them to, rather than more mature children who had outgrown the need for a nap.
For the study, psychologists went into pre-school classrooms and taught 40 children aged three to nearly five years old a simple computer game. It required them to memorise the positions on a grid of images including a cat, an umbrella, and a policeman. The children were trained from 10am until they could remember the positions of around 75% of the pictures.
The scientists visited each child twice over the course of the study. On one visit, the child slept for an hour or so between 1pm and 3pm, and stayed awake on the other. To see how sleep affected their memory, the scientists tested each child again later the same afternoon.
Those who napped saw no change in their morning score of 75%, but the ones who stayed awake fared much worse, averaging scores of 65%, according to a report in the Proceedings of the National Academy of Sciences.
"If they stayed awake they forgot more of the items they had remembered in the morning, whereas if they took a nap, they remembered all the items they had learned in the morning," said Rebecca Spencer at the University of Massachusetts in Amherst.
To check whether the nappers were simply more alert and attentive than the others, the scientists returned the next day and tested the children again. Their scores did not change, suggesting that daytime sleep was crucial.
Spencer said that parents and pre-schools are steadily cutting down the time given to children's naps to fit a convenient schedule, or to make more time for a curriculum.
"You often see children forced out of napping, and hopefully this will help parents and pre-schools to understand that maybe that's not the best decision, and that a nap is an important part of the day," said Spencer.
"This is the science that's needed to preserve nap time. If our goals are to prepare children for early education, the naps are consistent with that goal because it's really helping them to learn," she added.
The scientists went on to look at the brain activity of 14 children while they slept in the unversity's sleep lab. They found that children with the best memory recall experienced more "sleep spindles" – brief bursts of activity thought to happen when the brain shunts memories from short-term storage in the hippocampus to the neocortex.
"This demonstrates very vividly the value of sleep for consolidating memories. Very young children find it quite difficult to retain information over the course of a full day, so in those circumstances, a nap is going to be useful for ensuring that whatever they learn is retained in the longer term," said Gareth Gaskell, professor of psychology at York University.
"At these ages, children are shifting from multiple sleeps in 24 hours to a single sleep overnight. It may well be that non-habitual nappers are becoming more adult-like, and have got that little bit further in the process."