Sleep work
For many years the purpose of sleep has puzzled thinkers. In the past decade, one role has become increasingly clear: it helps us make memories.
However, there are different forms of memory and different types of sleep, so much remains to be learned. But many fascinating discoveries have been made recently.
Take a task in which subjects had to type a series of numbers - say, 2-4-7-2-4-7 - as accurately and quickly as they could. After practice, subjects got quicker and more accurate. If retested 12 hours later, they were no better, but after a night's sleep they were 20 per cent faster and their error rate fell by 36 per cent. Even a 90-minute nap led to a 16 per cent improvement in speed. A second night's sleep improved performance even more.
In a test of motor skills, where subjects had to draw a line on a tablet PC while the computer tried to push the line aside, sleep again led to better performance. Also, during deep sleep there was an increase in the amount of brain activity specifically over a part of the brain involved in the task. If this pattern was blocked, no improvement in performance was seen.
So it seems that the regions of the brain needed for a task are active while we sleep, preparing themselves to perform the task when we wake up. (A similar thing seems to happen when we are awake and restful.)
And it is not just simple learning tasks that benefit from sleep. Complex activities such as dancing also improve, and people get better at mind-bending puzzles.
In one remarkable experiment, subjects were taught a complex mathematical technique to which there was actually a simple alternative. When tested again 12 hours later, the likelihood that they discovered the alternative more than doubled if they had had a night's sleep, even though they did not know there was an alternative to discover.
Sadly there is less evidence that sleep enhances memory for facts or for events. In rats, sleep seems to enhance spatial memory - rats are better in mazes after a night's sleep. Rapid eye movement (REM) sleep seems to be important here - and may also be important in enhancement of emotionally charged memories in humans.
Although not certain, it appears that we need sleep so that the brain can sort itself out without interference from conscious activity. We may not know it, but we wake up better prepared for the world than when we went to sleep.