But come nightfall a group of friends from the Calder Valley change beyond all recognition.

Their idea of playing out involves coming home covered in mud with more than the odd black eye or bruised limb thrown in for good measure. Last week one of them ended up in hospital having her calf stitched.

But it's risking life and limb and pushing themselves to the limit that gives this group, who range in age from 37 to 48, the adrenaline rush they crave.

Kirsty Law, Penny Sheard, Heidi Fletcher, Jane Hodgson, Gill Davies, Janine Oldfield, Mandy Halstead and Helen Sunderland are becoming known locally as "Birds On Bikes".

Every Monday evening, come rain or shine, they can be found on their mountain bikes pounding the rugged terrain of the Yorkshire moors. This is no ordinary mountain biking, however, and these are no ordinary women.

Their sport is known as "night riding" in mountain-bike circles and is as dangerous as it gets. They fly up and down hills in the pitch black with only a headlight to guide them and say while it might be foolhardy, it's also exhilarating, exciting and a great way to relax and get rid of stress.

"The group, who include a barrister, solicitor, company director and educational psychologist, live for the thrill of Monday nights to help them cope with their hectic lives. They leave their children and partners behind them for what they call their "playing-out night".

What started off as a one-off bike ride has now become a weekly fix for them, says 42-year-old Heidi, a law reporter with a legal publishing company. She says while she hated the idea of wearing padded shorts at the beginning, they are now a must because the rough terrain can be quite painful on the buttocks.

"I agreed to join Kirsty and Penny for a one-off ride. My partner is a keen bike rider so I borrowed his lights.

"The rest is history. It's exhilarating to ride at night. And it makes going uphill that much easier – you can't see how steep it is," she says.

Helen, a 45-year-old PA, says there's nothing to beat the thrill of flying down a hill in the dark with the wind on your face, but admits even familiar descents become more treacherous in the dark, and she should know because she fell off her bike last week and ended up in hospital.

Kirsty, 37, a personal trainer and 37-year-old Penny, an educational psychologist, go way back and are experienced riders. They compete in the annual Polaris Mountain Bike Orienteering Challenge, a two-day event that combines mountain biking with navigation and touring. Janine Oldfield, a 38-year-old school bursar is also an experienced rider.

The women take it in turn to suggest Monday night routes and can be out for between two and four hours. "It's a fantastic way to really get to know the area," says Gill, a 48-year-old marketing director who is the only one who doesn't live locally. She used to live in Hebden Bridge, but now makes a 100-mile round trip from her home in York every week.

Getting wet, dirty or injured is all part of the fun say the group, and the fear of falling off is often greater than the fall itself. Mandy, 42, a catering instructor in a male prison, was left in agony with severe bruising after a particularly painful fall, but it wasn't long before she was back in the saddle. And Penny claims the only time she ever completed a perfect cartwheel was when she went flying over the handlebars straight on to her feet!

"We don't see ourselves as doing anything radical, although we do get comments from workmates and other friends who really can't understand why we want to push ourselves so hard and get so dirty," says Jane, 41, a barrister.

Most of their partners are bike enthusiasts so how do they shape up? "We might not be as tough as the guys but we are definitely fitter and have a lot more stamina. They tend to give up while we keep going," says Heidi.