Better known as SAD, the acronym doesn't always help. It's one of the best shorthands in psychiatry, Dr Laurence Wainwright, a psychiatry academic at Oxford University, tells Sky News - but it also makes it easy to pass off as "just" sadness. Actually, it is a "major depressive disorder", he says. But because it comes and goes, it's not taken seriously - even though it can be "harrowing" for sufferers.