Revealed: why we need a good night's sleep

Scientists warn of the dangers of sleepless nights after discovering how brain clears out day's mental rubbish

A study into slumber has found that the nerve connections built up in the brain during a busy day are pruned back during the night in an attempt to keep the mind from overloading on junk information.

The findings lend support to the idea that a good night's sleep is essential for consolidating important memories of the previous day and getting rid of things that would otherwise clog up the system.

The researchers behind the study said the results showed how important it was for people to get a good night's sleep to be on top form the next day.

"Right now, a lot of people are worried about their jobs and the economy, and some are no doubt losing sleep over these concerns," said one of the authors, Paul Shaw, from the Washington University School of Medicine in St Louis.

"But these data suggest the best thing you can do to make sure you stay sharp and increase your chances of keeping your job is to make getting enough sleep a top priority."

The research was based on analysis of fruit flies. The scientists believe that these simple creatures are a good model of sleep in humans because, like people, flies need between six and eight hours a night and show physical and mental signs of deprivation if they fail to get enough.

Like humans, fruit flies exposed to a busy day's activities need more sleep. Those raised in crowded conditions sleep two or three hours longer than those kept in solitary confinement and flies that are kept active with "mental workouts" sleep longer than those that are not.

Previous studies had shown that sleep promotes learning and memory in animals. The latest research went further by showing that the connections (synapses) between nerve cells in the brain are built up during the day and are pruned after a good night's sleep, Professor Shaw said.

"There are a number of reasons why the brain can't indefinitely add synapses, including the finite spatial constraints of the skull. We were able to track the creation of new synapses in fruit flies during learning experiences, and to show that sleep pushed that number back down," he said.

The scientists kept the fruit flies in a specially designed "fly agitator" with a robot arm that occasionally shook the resting platforms for the flies to prevent them from sleeping. The scientists found physical changes in the brains of these sleep-deprived flies – specifically, a build-up of the proteins that connect one nerve cell to another.

When the scientists analysed the brains of the flies after they had slept, the level of these proteins had dropped, indicating that the nerve connections themselves had become weaker or even eliminated during the night.

"At the end of sleep, the strongest synapses shrink, while the weakest synapses may even disappear," said Chiara Cirelli, from the University of Wisconsin-Madison, another author of the study, published in Science.

"Much of what we learn in a day, we don't really need to remember," she said. "If you have used up all the space, you can't learn more before you clean out the junk that is filling up your brain."