Baby birds and mammals sleep for up to three times as long as adults. New research suggests that this may be to allow their brains to adapt to the riot of novelty they experience.

Michael Stryker, of the University of California, San Francisco, found that the amount of change in the brains of young cats placed in a new environment was tightly linked to the amount of slow-wave sleep -- the non-dreaming state -- the cats got1.

This link is a surprise, says Stryker, as dreams, also known as rapid eye-movement (REM) sleep, are often thought of as the brain's way of replaying and analysing the day's experiences.

"The finding that the benefit of sleep seems to lie in non-REM sleep is very interesting," says Jim Horne, a sleep researcher at the University of Loughborough, UK. "REM sleep has been seen as important in the maturation of the brain."

Chemicals released during sleep might remodel the brain by stimulating and inhibiting neural growth, Stryker says. Many hormones, such as testosterone and growth hormone, are released at night, and he suspects that nocturnal chemical signals "may really play a crucial role in consolidating and enhancing waking experience".

The team studied young cats, whose brains are developing rapidly and are particularly sensitive to environmental change. The researchers covered one of the cats' eyes for six hours. This causes the brain's visual cortex to weaken its response to the image from the covered eye, and up its commitment to the active eye.

When cats slept for six hours after the test period, the amount of change in their neural responses was double that of cats denied a nap1. Sleepers' brains changed more even than those of cats that spent an extra six hours with one eye covered.

Might this be a general function of sleep? Can we extrapolate from developing cats to fully formed humans, and from the visual system to other forms of mental adaptation?

Stryker thinks so: "We wanted to ask whether, for example, studying for an exam and then sleeping is as effective a way of learning as pulling an all-nighter." These results, he says, suggest that the route to an A-grade passes through the Land of Nod.

Psychologist Carlyle Smith, of Trent University, Ontario, Canada, studies how sleep affects human learning and memory. He agrees that similar neural changes might occur in the sleeping human brain after new experience.

In his experiments, Smith too has seen a link between slow-wave sleep and people's ability to learn repetitive but delicate and precise patterns of movement -- of the type that "might be important for sports people or musicians".

"If we train people on a task, we find that if we interrupt their [slow-wave] sleep, they don't remember how to do it," Smith says.