Over-40s perform best if their working week is restricted to three days, researchers have found. Experts in Japan compared the hours people worked with the outcomes of a cognition test for 3000 men and 3500 women over 40 in Australia.Results suggest that a part-time job is the best for keeping the brain stimulated.Participants were asked to read words aloud, recite lists of numbers running backwards and to link letters and numbers in a particular pattern under time pressure.The researchers found that people who worked about 25 hours a week got the best test scores.Colin McKenzie, professor of economics at Keio University, Tokyo, said the evidence indicated that working too much or too little could cause a decline in cognitive performance."Work can be a double-edged sword in that it can stimulate brain activity, but long working hours can cause fatigue and stress and damage cognitive functions," he said. ..