An hour's nap in the afternoon can boost a person's brain power and improve their memory, according to a study showing that short periods of sleep during the day can make it easier to function mentally.
Scientists found that a Spanish-style siesta after lunch does more than just refresh the body and mind, it also makes it easier for the brain to store and retrieve items of short-term information needed for working or studying.
The findings lend weight to the idea that sleep not only restores a person's sense of well-being, but is essential if the brain is to take on additional information as part of the memory-forming process of learning.
"Sleep not only rights the wrong of prolonged wakefulness but, at a neuro-cognitive level, it moves you beyond where you were before you took a nap," said Matthew Walker, a psychologist at the University of California Berkeley.
The study took 39 healthy volunteers who were divided into two groups. At noon, both groups took part in a series of rigorous learning tests intended to tax a region of the brain called the hippocampus, which is known to be involved in the formation of short-term memory.
One of the groups was then asked to take a 90-minute nap at 2pm, while the other group stayed awake. Both were then asked to take part in a subsequent set of tests at 6pm to see how well they could continue learning.
Those who had remained awake during the afternoon performed significantly worse in terms of learning ability at 6pm than those who had taken the nap.
The people who had slept not only did better, they actually improved their capacity to learn, Dr Walker told the American Association for the Advancement of Science in San Diego.
The results appear to support earlier work suggesting that fact-based memories are temporarily stored in the hippocampus before being sent to the brain's prefrontal cortex, which may have more "storage space", he said.
"It's as though the email inbox in your hippocampus is full and, until you sleep and clear out those fact emails, you're not going to receive any more mail. It's just going to bounce until you sleep and move it into another folder," Dr Walker said.
The study also found that the kind of sleep that makes learning easier was the type of deep sleep where there is no "rapid eye movement" (REM), when the eyelids flicker during the dreaming phases of night-time sleep. Non-REM sleep appeared to be essential for the brain refreshment that improved learning, Dr Walker said.
"I can't imagine Mother Nature would have us spend 50 per cent of the night going from one sleep stage to another for no reason. Sleep is sophisticated. It acts locally to give us what we need," he added.
A separate study found that sleep is also important for learning in babies. Infants who had napped were better at generalising their knowledge of spoken words than infants aged 15 months who had stayed awake during the study period, Professor Lynn Nadel of the University of Arizona told the meeting.
It is likely that infants at that age mostly have REM sleep, and, unless they can sleep soon after learning the construction of words in a sentence, then they are unlikely to remember it.
"What we know is that infants have mostly REM sleep ... and they have to get some of that sleep within a reasonable amount of time after inputting information in order to be able to do abstracting work on it. If they don't sleep within four to eight hours, they probably lose the entire thing," Dr Nadel said.