Jenny Colgan is one of Britain's most prolific writers. Last year she wrote five books and this year eight are scheduled to come out. To produce this volume of work you might think Colgan, 44, wrote into the small hours every night. She doesn't. She works for no more than three hours every day, from 11am to 1pm. Like a marathon runner building up resistance I started to push how much I could get into every day. And every time I stretched it a hundred or so here and there, I found that I could, even though the time I have for working stayed about the same. "Weirdly, the work started getting better. I'm now finishing my novels more quickly, immersing myself more, focusing better. The arcs of the books, the reviews and the sales all improved massively." This will come as no surprise to Colin McKenzie and his team at Keio University in Japan, who has just published a paper suggesting that part-time workers over the age of 40 - especially those who work about 25 hours a week - have the sharpest brains. Part-time work, the report has concluded, is the perfect balance between brain stimulation and stress. The findings echo those of a celebrated study that has followed 10,000 middle-aged civil servants in Whitehall since 1985. In short, working too hard is bad for you. The report's title is "Use it too much and lose it?" It has been welcomed by a host of people who have called for Britain to end its culture of hamster-wheel offices.