1.20am: John Ayre is working out at Swindon's Primebodies gym. At a time when the streets are usually given over to drunks and street-sweepers, he - and a growing number of nocturnal trainers - head for the gym. What better snapshot of our growing national obsession with fitness than the small-hours workout? Is there now no downtime? Or, perhaps, the ever-active treadmill is a natural consequence of the open-all-hours society...

When it opened in May 2002, Primebodies was one of the first 24-hour gyms in the UK but it has since been joined by several more. Those taking advantage of its opening hours range from IT consultants to factory workers. 'I'm a self-employed butcher and I work long hours,' says Dave Williams. For him and many other shift workers this gym is ideal - they might finish a shift towards midnight and are able to go lifting weights straight from work. Williams also likes the social side of training in the small hours. 'You get to know the small regular crowd,' he says.

According to the Fitness Industry Association, since 1996 there has been a 25 per cent increase in the number of gyms and health clubs in the UK, with a current total of 2,600. An astounding 40 per cent of the British population are now gym users or members, according to a recent poll, and over £1.25 billion is spent annually on membership. That means more people are now gym-goers than church-goers. And, if the boom continues, so opening hours will expand.

The world's largest gym group, US-based 24hr Fitness, has proved the popularity of all-hours workouts with over 300 gyms and 2.7m members worldwide. (Germany in particular has taken to the 24-hour gym.) Ayre, hitting the treadmill in the small hours, sums it up: 'I like the relaxed atmosphere of the gym at night. It beats sitting in front of late-night TV.'