The 24-hour society is shortening lives and harming health because of the havoc it plays with our biological clock, scientists claim today.

The 24-hour society is shortening lives and harming health because of the havoc it plays with our biological clock, scientists claim today.

One in five workers in urban societies is now working outside normal office hours in response to the increased demand from consumers for round-the-clock access to shops, banks and other services.

But convenience for customers comes at a price paid in terms of increased sleep disruption, gastro-intestinal disorders and risk of heart disease among those who work unsocial hours.

Shantha Rajaratnam and Professor Josephine Arendt, from the Centre for Chronobiology at the University of Surrey, warn that the increased health risks will lead to more litigation unless employers use current scientific knowledge about how to help employees adapt their circadian rhythms.

Writing in The Lancet, the authors cite the case of a woman in the United States killed in a road accident when a tractor-trailer crashed into the back of her car. Her family received a $24m (£16m) settlement from the driver's employer after lawyers argued in court that the employer had violated regulations governing hours of work, resulting in the tractor-trailer driver falling asleep at the wheel.

In addition to dangers caused by sleepiness, pressure on employers to reduce increased risks from heart disease and other health problems are likely to increase, they say.

Carefully timed exposure to light, which may include the use of dark goggles for night workers during the day, and selective use of the hormone melatonin can speed adaptation to new hours of work, or new hours of daylight for intercontinental travellers. Melatonin is made by the pineal gland in the brain and appears to be important in initiating or maintaining sleep. It is available as a drug and in countries where it is freely available (it is not licensed in the UK) it is widely used as a treatment for jet lag and as a sleeping pill.

But the authors warn there is "little evidence for optimum dose or formulation" of melatonin and "no information on long term safety."

They write: "Biological time is not only scientifically important, but it also greatly affects the productivity and health of a nation. The cost to the nation's health of working out of phase with our biological clocks is probably incalculable at present ... Employers and individuals need to be aware of the major performance and alertness decrements associated with night activity and how to best manage and counteract them."

In Britain, working times regulations introduced in 1998 limited hours of work but did not take account of the effect of shift patterns on mental alertness. The authors say: "In view of the risk the increased risk of accidents during early hours of the morning, greater regulation of work practices during these times is warranted."

The body's biological clock generates and maintains circadian rhythms in temperature, blood pressure, sleep and wakefulness, mental performance and the synthesis of certain hormones.

Trying to sleep at the "wrong" phase of the circadian cycle will usually lead to a disturbed "night" with frequent awakenings, resulting in the exhaustion seen in jet lag and among shift workers.