Remember the last time you had a night's sleep?

By STEVE CONNOR

Burning the midnight oil before exams or interviews can drive knowledge away, says research that suggests sleep is needed to allow memories to be "downloaded" into the brain.

A good night's sleep within 30 hours of trying to remember a new task is necessary for effective recall in subsequent weeks, scientists have found.

The findings, published in the December issue of Nature Neuroscience, showed that it was the act of sleep, rather than the simple passage of time, that was critical for long-term memory formation.

"We think that getting that first night's sleep starts the process of memory consolidation," said Robert Stickgold, a sleep researcher at Harvard Medical School, who conducted the study.

"It seems that memories normally wash out of the brain unless some process nails them down.

"My suspicion is that sleep is one of those things that does the nailing down."

About one in five people say they are so short of sleep that it affects their daily life.

But the study highlights a less understood side-effect of insufficient sleep, serious memory impairment.

Volunteers found it easier to remember a task if they were allowed to sleep.

For others who were kept awake, no amount of subsequent sleep made up for the initial loss.

Professor Stickgold's team invited 24 volunteers taking part in the study to identify the orientation of three diagonal bars flashed for one-sixtieth of a second on a screen that was also covered with a series of horizontal stripes.

Half the subjects were allowed a night's sleep and half were kept awake, but both groups were later allowed sleep on the second and third nights, to make up differences in fatigue between the groups.

The tests found that those who slept on the first night were significantly and consistently better at performing the memory task.

The second group showed no improvement, despite their catch-up sleep over the two subsequent nights.

Other research at the Medical University at Lubeck in Germany showed that memories are laid down in two stages during the night, first in the deep "slow wave" sleep that occurs in the first half of the night, then in the less important stage of dreaming, or "rapid eye movement" (REM).

If people are deprived of sleep during the first half of the night, their capacity for memory consolidation appears to be almost the same as if they had had no sleep at all.

Being deprived of REM was found to be less detrimental to memory, yet important in "implicit learning," in which performance does not depend just on the ability to remember.

The scientists concluded that a good night's sleep, which included both slow-wave and REM sleep, produced the most effective performances among the volunteers.