A review in press in Consciousness and Cognition discusses evidence for the "incubation effect", which refraining from conscious thought about a problem can result in a more creative response. Despite abundant anecodatal support for this idea (Einstein cutting himself while shaving, etc.) it has been very hard to demonstrate in a lab.
The authors are interested in the very first phase of creative thinking, the generation of new and original thoughts. They designed experiments which allow for open-ended answers.For example, 3 groups of people were asked to list "towns that begin with A." One group was allowed to begin immediately; one group was allowed to think about the problem for an interval; and a third group was given a highly distracting test for an equivalent time before returning to the question. The authors scored the "originality" of the lists based on the population size of the municipalities, with big, prominent towns considered a less original answer than small villages.
Every group managed to come up with a fair list of towns. Both delayed groups tended to give more total answers than the group asked to respond immediately.
The most dramatic effect, a nearly twofold higher number of small villages named, is seen between the group asked to answer immediately and the distracted group. The group which was allowed to consciously consider the problem got intermediate scores, although tending toward the immediate answer group. There is also a small tendency for the distracted group to come up with *fewer* "obvious" answers in their list.
My final reaction to this is that it's very interesting to see effects with such a simple experimental design. I myself feel refreshed and sharper especially after a vacation, so I'm inclined to believe there is a real cognitive effect which just needs careful experimental design to be teased out. I think I'd like to see the concept tested using a bigger variety of tasks (they always asks for written lists) and larger sets of people.
Update: there seems to be a fairly extensive literature all suggestive of a positive effect of being a way from a problem for a while.