A few months ago, my friend asked me if I wanted to sign up to a \"fit into your old jeans\" weight-loss challenge being organised by her gym. The posters promised that you would drop a dress size in six weeks, provided you turned up five times a week and followed a no-carb diet. Although the closest I got to exercise was lifting the remote control, I signed up. I didn\'t have just one pair of jeans I couldn\'t fit into, I had a drawer full. The first week was terrible. After my first session of trying to do exercises with silly names such as \"jumping jacks\" and \"burpees\", I left the gym in shock. But on Day Ten I was weighed, and was told that I had three kilos and 30ml of fat — that\'s about two tablespoons. My trainer said people usually aim to lose about 2ml of fat a week. At home I took out the tape and I had lost two inches from my waist. I was back on board. And then an amazing thing happened. Instead of leaving the gym grumbling, by the end of Week Three my aches had stopped and I was enjoying exercise. But it wasn\'t just the physical benefits. Usually I wake up at least a couple of times in the night, my mind racing, but after a few days in the gym I was sleeping for ten hours. What\'s more, I was waking up full of beans, jumping out of bed to start the day. I was also becoming happy — ridiculously, singing-to-myself happy. And I was so productive. Work that I would fret over for a week got done in two post-gym hours. And it\'s not just me. More studies are now finding exercise has a much greater effect on our lives than just helping us to keep trim. While we all know regular exercise can help prevent cancer, heart disease, diabetes and obesity, research is now finding that exercise also makes us happy — and nicer. Regular exercise could even help you do better at work. In November a study at Leeds Metropolitan University found workers who used their company gym were more productive and happy. Professor Jim McKenna surveyed 210 office workers who used their company gym and asked them to rate their frame of mind and work performance at the end of each day. He compared the results on days they exercised and days they didn\'t. Sixty-five per cent of people said they managed their time better, were more productive and got on better with their colleagues on the days they exercised. The connection between exercise and workplace productivity is so well recognised in the US that some companies have set up conference rooms with \"treadmill desks\". The idea is that employees are better able to communicate when they are exercising. Other offices encourage workplace workouts, while some Chinese businesses enforce twice-daily exercise breaks in which employees have to move around. It sounds bizarre, but might we all be following suit soon? Personal trainer Jean Claude Vacassin says: \"I do a lot of work with corporations, talking to employees about fitness because HR departments are realising how important it is for staff productivity.\" Exercise produces feel-good endorphins that result in a feeling of euphoria immediately afterwards and a greater feeling of calm throughout the day. Indeed, research has shown that exercise can be as effective as antidepressants in treating mild to moderate anxiety and depression. Compare this with the effect of my usual regime of hours slumped on the sofa watching television — which increases your chance of becoming depressed by 13 per cent, according to a Harvard study — and the difference is like night and day. So yes, at the end of the six weeks, I did get what I wanted. I got into my skinniest jeans, could see my jawline for the first time in years and dropped from a size 14 to a size 12. Getting fit changed my life. However, that was two months ago and since then there has been a month of Christmas excess and a long holiday, which means the inches have gone back on. But I don\'t mind. Weight will come and go, but the joy I find in exercise is going to stay.