Observationally I think there's a huge exercise component, in past centuries miners, textile/sweatshop workers, and assemblyline workers didn't seem to suffer.
That's an interesting theory. Data about union votes to start/end strikes vs month of year vs latitude could show some correlation where union outdoor worker hotheads in June will strike till they win but in January some important percentage of them would be more likely to give up. I don't think the data shows a correlation but a more serious attempt at research might find something.
