How much proper brainwork - not zoning out in meetings, or reorganising the stationery cupboard, but work that involves really thinking - should you aim to get done in one day? The answer isn't some sophisticated version of: It depends." The answer is four hours. That, anyway, is the persuasive conclusion reached by Alex Pang in his book Rest: Why You Get More Done When You Work Less. This column has evangelised before about the truth of that subtitle, what with the nine-to-five being a relic of the industrial revolution with no relevance to modern "knowledge work" - but what's so striking about Pang's argument is its specificity. Ranging across history and creative fields, he keeps encountering the same thing. Charles Darwin worked for two 90-minute periods in the morning, then an hour later on; the mathematician Henri Poincar from 10am till noon then 5pm till 7pm; the same approximate stretch features in the daily routines of Thomas Jefferson, Alice Munro, John le Carr and many more. And maybe it's not just creative work. Half a century ago, the anthropologist Marshall Sahlins caused a stir by suggesting that people in hunter-gatherer societies aren't ceaselessly struggling for survival; on the contrary, they'd built "the original affluent society", by keeping their needs low, then meeting them. Crunching numbers from Africa and Australia, he calculated the average number of hours hunter-gatherers must work per day, to keep everyone fed. That's right: it was "three to five hours". Don't you think it's time we took the hint?