I WAS on a train travelling back from Manchester when a man wearing a hipster flat cap sat down opposite me. He was in his fifties, wearing clothes that would have looked better on someone 20 years younger, and was chatty. Very chatty. Now normally, I would be more than happy to while away an hour talking about the price of fish with a complete stranger, but not on that day. Having barely slept the night before, I was desperate to catch a few winks on the six-hour journey. Unfortunately or me, the gentleman started to talking to me. And he didn’t stop.

Effusively friendly, and apparently oblivious to my attempts to nap, he told me tales of birthday parties, his travels across Europe, presents his son had bought him, and plans to attend a rock concert in Switzerland. Barely pausing for breath, it was a barrage of witterings that felt like sitting in front of a full-volume television without the remote control to turn it off.

An eccentric man? He certainly was – but I sensed there was something more. Within his cheerful ramblings were mentions of family arguments and ‘sectioning’ – a term that means to be forcibly detained in a mental health care facility. After squeezing in a few questions of my own, it become obvious that although he appeared to be full of joy, he was in fact deeply unwell. This affable man was suffering ‘mania’ – a profoundly positive mood swing that is part of bipolar disorder.

Of all mental health issues, bipolar disorder is one of the least understood by the general population. It used to be called ‘manic depression’ and is surprisingly common, affecting one in every 50 people at some point in their life. It runs in families and has affected the likes of Ruby Wax, Stephen Fry, Frank Bruno, Russel Brand and Spike Milligan. Many people think that bipolar is like having mood swings. “I think I’m a bit manic-depressive,” I hear some people say, but the real condition is far more devastating.

We all go through ups and downs, but for people with bipolar the mood gradually shifts up or down over days and weeks. It can steadily slump into a severe depression or ascend into excessive joyfulness, termed mania. If mood swings are rapid, over the course of a day or two, then it usually isn’t bipolar. And if you think being permanently cheery is a good thing then think again. Entering mania can mean losing grip on reality. Thoughts race, sleeping stops, you overestimate your abilities, take dangerous risks, make poor decisions and spend money – possibly running up huge debts. Sometimes, people who are going through a manic episode believe that they are famous, chosen by God, or even are God themselves.

Teasing apart, what this euphorically happy man was saying, it seemed, was that he had suddenly left home with his wife’s car keys, hitch-hiked across the country and planned to take a ferry to the continent, for no apparent reason. Like many people who experience mania, he was oblivious to his dangerous state of mind. Bipolar disorder is very treatable with ‘mood stabilising’ drugs, although the side effects can be unpleasant and few people with mania like being ‘brought down’. Excusing myself (“to go to the toilet”) I managed to tell a train guard that I was concerned for him. However, when I returned to the table he had vanished. In my absence, I think he had jumped off the train when it stopped at a small rural station. I will probably never know what happened to him, but I can only hope that he met someone else who knew about bipolar disorder and could guide him towards help.