watertank | from the New Scientist mag (Reply)

While you slumber, your brain puts the world in order

Bob Stickgold from Harvard Medical School and his colleagues found that people were better able to recall lists of related words after a night's sleep than after the same time spent awake during the day. They also found it easier to recollect themes that the words had in common - forgetting around 25 per cent more themes after a waking rest. "We're not just stabilising memories during sleep," says Stickgold. "We're extracting the meaning."

In another experiment, people were shown cards with symbols followed by reports of various weather outcomes - so for example, diamond shapes might be followed by rain 70 per cent of the time. Twelve hours after training, people felt able to guess the weather from the symbols, though they struggled to voice their "rules". After sleeping, their predictions were 10 per cent better.

Sleep helps us extract rules from our experiences, says Stickgold. It's like knowing the difference between dogs and cats, he says, even if it is hard to explain. He presented the results last week at a meeting organised by The Science Network at the Salk Institute in La Jolla, California.
From issue 2592 of New Scientist magazine, 22 February 2007, page 17

also relates to http://watertank.livejournal.com/663559.html
What I find necessary for efficient "unconcious" brainwork is arming your mind with the right set of euristic tools. Then it somehow figures out which tools to use at the right time. Just "sleeping on it" works only for relatively simple problems.