In his meticulous diaries, written from 1846 to 1882, the Harvard librarian John Langdon Sibley complains often about the withering summer heat: “The heat wilts & enervates me & makes me sick,” he wrote in 1852. Sibley lived before the age of air-conditioning, but recent research suggests that his observation is still accurate: summer really does tend to be a time of reduced productivity. Our brains do, figuratively, wilt.
One of the key issues is motivation: when the weather is unpleasant, no one wants to go outside, but when the sun is shining, the air is warm, and the sky is blue, leisure calls. A 2008 study using data from the American Time Use Survey found that, on rainy days, men spent, on average, thirty more minutes at work than they did on comparatively sunny days. In 2012, a group of researchers from Harvard University and the University of North Carolina at Chapel Hill conducted a field study of Japanese bank workers and found a similar pattern: bad weather made workers more productive, as measured by the time it took them to complete assigned tasks in a loan-application process.
When the weather improved, in contrast, productivity fell. To determine why this was the case, the researchers assigned Harvard students data entry on either sunny or rainy days. The students were randomly assigned to one of two conditions: before starting to work, they were either shown six photographs of outdoor activities in nice weather, such as sailing or eating outdoors, or were asked to describe their daily routines. The researchers found that participants were less productive when they’d viewed pleasant outdoor photographs. Instead of focussing on their work, they focussed on what they’d rather be doing—whether or not it was actually sunny or rainy outside (though the effect was stronger on sunny days). The mere thought of pleasant alternatives made people concentrate less.
For the rest of the story: http://www.newyorker.com/online/blogs/elements/2013/07/psychology-why-summer-makes-us-lazy.html