At 29 minutes past four next Monday morning, postman George Ross will stretch across his bed and turn off his alarm clock before it rings. Ross will already have been lying awake in his bed for the previous hour or so, as his body won't have acclimatised to the clocks going back an hour. Meanwhile, across the country, countless owls will be waking at a time they consider to be more humane than usual.

Professor Derk-Jan Dijk, director of the Sleep Research Centre at the University of Surrey, says the clocks changing back to Greenwich Mean Time affects both morning and evening types equally: "I would expect that, because we get to sleep that extra hour, the morning person would be wide awake long before the alarm clock goes off. The evening person will think they are finally waking at a more or less a reasonable time."

Just as there is a range of heights across humanity so there is a range of sleep types, pre-programmed by our biological clocks, with some people's simply running faster than others. While the structure of the traditional working day suits morning types, evening types only start functioning properly later on. "Our biological clock is our internal time-keeper," says Dijk. "It gives us a system to know when it is time to wake up or go to sleep."

The biological clock is driven by three quantifiable rhythms. The first is the rhythm of melatonin, a hormone produced at night which regulates sleep-wake cycles. The second is the rhythm of cortisol, a hormone that regulates blood pressure. (This hormone is at higher levels in the morning, peaking just before wake time, and it drops in the evening.) The third component, meanwhile, is the core body temperature, which drops to its lowest internal temperature about four hours into sleep.

Researchers have found there are demonstrable physiological differences between the two extreme types, because we all respond differently to the hormones in our system: "We find that in morning types the rhythms are phased or timed two to three hours earlier," says Dijk. "Cortisol peaks at just around waking time and that peak occurs earlier in the morning type than in the evening type. It is cortisol surges that will help your physiology prepare for all your waking activities."

Studies suggest the biological clock was an important part of primitive man's survival; anticipating the changes from day into night and from one season into another helped early humans maximise their energy levels. It suited some - hunters, perhaps - to be nocturnal beings.

Interestingly, whether you are a lark or an owl can change over a lifetime. Research suggests that the older we get, the more of a morning type we are likely to become. No one is sure why, but one theory is that from birth to death, there is a decline in how much sleep we need. If an older person continues to go to bed at the same time, they will tend to wake up earlier - and will tend to be kept awake by the light coming through the window.

But another school of thought puts the change down to nothing more than your sleep system not working as well as it used to, in just the same way that joints stiffen and hair falls out.

However, the change in sleeping patterns as we age can bring psychological problems if there is a mismatch between how much sleep you actually need, and what you think you need. The trick, says Dijk, is to impose your own sleep needs on the day, rather than being too hidebound by the idea that you've always gone to bed at a certain time and woken up at a certain time, and will continue to do that decade after decade.

Many of the patients referred to Dr Chris Idzikowski, director of the Edinburgh Sleep Centre, have problems matching when they think they should be waking up to their biological alarm clock. They attempt to alter their behaviour through heavy use of artificial stimulants to better fit the structure of a typical working day. However, Idzikowski believes the solution is simply one of acceptance. "If people understood their own internal clocks better, they could avoid problems," he says.

A few years ago he had a couple of patients who were at either extreme of the sleep type range. One would go to bed at nine in the evening and wake up at three in the morning while the other would go to bed at three in the morning and get up at nine. He adds: "They were both perfectly happy - they just wanted to check whether it was OK."

Idzikowski has found that more successful owls find work that they can structure around their sleeping habits: "They end up being self-employed or working in a creative environment where they can handle their own timetable."

Derek McFarlane is an owl who has had to struggle to fit his sleep patterns into the nine-to-five template. An IT technical analyst, who gets by on six hours sleep a night midweek, he struggles to get up at 8am and is late for work a couple of times a week: on one occasion he even slept in until past noon.

He said: "I survive on a low amount of sleep during the week and catch up at the weekend by sleeping until 11 or 12am.

Ideally, he says, he would like to work from late morning through to mid-evening, but that is simply not possible with his current employer. At work he doesn't start functioning fully until about 11am and then has a dip about 4pm due to lack of sleep. However, he picks up again in the evening and stays awake until 1.30am, reading and listening to music.

So by the time McFarlane is beginning to feel awake in the morning, Ross will have already finished his postal delivery round in Aberdeen. He is a little annoyed that the changing clocks mean he will be starting work later than usual. He loves his early starts: in his ideal world, he would get up still earlier. "Even at the weekends, I only stay in bed an extra half hour or so," he confesses.