It is early morning in San José, California, and Elizabeth Safran, a consultant with a boutique Silicon Valley public relations company, is on her third cup of coffee and her umpteenth e-mail.

The 13-strong company is virtual: everybody works from home and relies on e-mail and instant messaging to stay in touch. Elizabeth worries about her work/life balance and thinks the pendulum has swung too far towards technology: "It makes us more productive but everybody is working all the time - weekends, evenings. It's almost overkill."

Over in the UK, it is late on Friday afternoon and Paul Renucci, managing director of systems integration company Damovo, switches off his computer. He has spent the day working at home and now it is time to pick up his children: "On Friday I work at home," he says. "I can work pretty hard but at 5pm exactly I stop working and the weekend starts. In the past, I would have had to brave the M25 [ London's notoriously traffic jam-prone ring road] and I would be lucky to be home by seven."

Safran and Renucci represent different facets of a modern phenomenon: the capacity of the latest communications technologies such as e-mail, text, instant messaging and video-conferencing to blur the distinction between work and leisure and to raise important questions about the nature of "flexible working" where employees have unprecedented freedom to work where and when they choose.

Thanks to devices such as the BlackBerry and the Palm Treo, "smart" mobile phones and Wi-Fi hotspots, e-mail is ubiquitous.

And while enthusiasts such as Renucci protest "there is always an off button" newly developed features such as "presence", which tells whether an individual is contactable or not, provide new tools with which employees' activities can be monitored.

There are three issues at stake. First, does the proliferation of portable, networked devices intrude on an individual's work and life in a burdensome, perhaps damaging, manner? Second, what is the effect of these devices on traditional workplace relationships? And third, how do individuals manage these devices without expiring beneath an avalanche of different, often incompatible systems?

It is an area that is beginning to attract serious research. Prof Lotte Bailyn of MIT's Sloan School of Management argues that rules and behavioural norms have yet to be established: "Nobody thinks about these issues when the technology first arrives. It's hard to think beforehand about norms and expectations but it is much more difficult to change these once they have become established.

"Within an organisation," she argues, "it is the top people that have to model the right behaviour. If they don't set the right example, then it's hopeless."

Something of a pattern can be discerned from the numerous surveys conducted into the effect of technology on work-life balance. When the technologies are introduced, e-mail and instant messaging seem to produce measurable improvements in efficiency and job satisfaction. One survey carried out by Visto, a US e-mail company, showed that four-fifths of mobile professionals were inclined to think that mobile e-mail would "enable them to work more flexibly by balancing a hectic work environment with the demands of everyday life".

It is a view shared by many senior executives. Roy Bedlow of Palm, the pocket computer group, claims e-mail enables him to stay in control: "Standing in a long queue to get through passport control at San Francisco airport, I was able to avoid the stress by going through all the e-mails I received during the 11-hour flight," he said.

But as the novelty wears off, some managers convince themselves that this capability is the same as availability. A recent Microsoft survey for YouGov, the UK market researcher, concluded there was: "The need for companies and individuals to take more active control of technology usage if we are to avoid becoming a 24-hour-a-day working society."

Nick Barley, a Microsoft executive, said: "There are lots of benefits in technology but for many that means more work, different times, the ability to be always contacted... Is this the price of modern living and do we have to accept this?"

If medium-sized UK businesses are a proxy for the rest of the business world, there is no doubt that pressure to increase productivity and technology are forcing change.

Microsoft found that "flexible working" had become a board issue for half of these companies and that only a third still claimed to have a nine-to-five culture.

The survey found that where flexibility had increased, so too had productivity and employee morale, coupled with lowered stress levels, absenteeism and staff turnover. It found little evidence in companies that espoused flexible working (it calls them "free range") that managers believed employees abused flexible working arrangements.

This latter finding is contentious. Inter-Tel, the Tucson-based telecoms group, for example, conducted a survey in Europe that found four in 10 employees did not think their employers would trust them to work at home while six in 10 thought that making a request to work more flexibly would damage their career prospects.

Back in San Jose, Ms Safran is mellowing: "The pendulum will swing back eventually and we will learn how to adopt these technologies which keep us connected a little more effectively," she says. Mr Renucci also thinks time will tell: "Kids today are constantly messaging each other about things that have happened to them during the day. As they get into the business community, they are going to expect to be working in those ways."