Undisturbed night's sleep needed for good memory

A continuous night's uninterrupted sleep may be the essential requirement for a good memory, according to a study carried out on laboratory mice. The animals failed to remember things after being repeatedly disturbed while they dozed. The research is more subtle than it first appears because the researchers did not just investigate disturbed sleep, which can cause memory loss through stress, but also the length of time the mice engaged in deep sleep, when memories are believed to be consolidated by the resting brain.

Many people who suffer from interrupted sleep, such as alcoholics and patients with sleep apnoea, caused by abnormal pauses in their breathing, often complain of poor memory. The study on mice may help to explain how sleeping brains are unable to consolidate their memories, scientists said.

It is not just sleep per se that is important for a good memory, but the length of continuous sleep, the study found. The experiments on sleeping mice "point to a specific characteristic of sleep – continuity – as being critical for memory," said Craig Heller, professor of biology at Stanford University Medical Centre in California.

Scientists have for many years believed that sleep is essential for a good memory but it has been difficult to tease apart the different factors involved. Waking humans and animals in mid sleep, for instance, can increase stress levels, which has been independently shown to affect memory formation, said Dr Luis de Lecea, associate professor of psychiatry and behavioural sciences at Stanford.

"Rodents are very sensitive to physical awakening. If you wake one up, it's going to be up for a while, and it will experience stress," Dr de Lecea said.

To get around the problem, the scientists used a kind of genetic modification that made certain nerves cells sensitive to light. These play a key role in switching between sleep and wakefulness, which meant that the scientists could use short bursts of light to fragment the animal's sleep without affecting the total time spent asleep or the quality of the sleep – whether it was deep sleep, for instance, or dreamy "rapid-eye-movement" sleep.

The study is published in the journal Proceedings of the National Academy of Sciences.