History repeats itself, the saying goes, but it was always hard to prove because it would involve analyzing a huge amount of data. However, by reviewing 87 years of U.S. and British newspapers, as well as postings on Twitter and visits to Wikipedia, researchers at the University of Bristol found proof of strong periodic patterns in people’s collective behavior. For instance, it turns out that the amount of time people devote to work or leisure depends on the weather and seasons, which was always suspected although difficult to prove. Cases of measles rise sharply in late March and early April. But other patterns that were not so readily noticeable also emerged, such as the correlation between seasons and mental health. Studying statistics of sentiments expressed by large numbers of people on Twitter, as well as how many times certain pages of the online encyclopedia Wikipedia were visited, researchers found that negative feelings were more prominent in winter, especially in November. Anxiety and anger are expressed more between September and April than during other periods of the year. The number of visits to pages describing panic disorder peaks in April. Scientists say the data shows that patterns of collective behavior affect people globally but are most pronounced in the northern hemisphere.