Shifts planned with little thought to ergonomics are a health-and-safety gamble. Fatigue, accidents, injuries and ill health are all but inevitable if the shift pattern doesn’t balance the demands of work with time for rest and recovery. Circadian Technologies, a London consultancy for 24/7 workforce issues, says the answer to reducing the risk is to allow plenty of time off – 11 to 16 hours – between shifts.
 
A study by the company reported in Business Week in April 2005 concludes that obesity, diabetes and heart disorders are higher for night workers, that they have a 20 percent greater chance of being involved in a severe accident and make five times more serious mistakes than their daytime counterparts.
 
One of several studies cited in the April 2005 issue of The Ergonomics Report™, "Shaping Night Work for a Better Fit With Workers," concludes that the risks can be reduced by offering workers enough time between night shifts to get the 8.4 hours of sleep recommended for good health. The publication, designed for subscribers with a professional interest in ergonomics, frequently covers human factors issues in the workplace.

Recent studies have shown that rest periods of 10 hours or less between consecutive shifts result in short sleep episodes, sometimes lasting only three to five hours. 

In an article published recently in Occupational Health & Safety, Circadian cites two studies that set the range for adequate time to secure the recommended 8.4 hours of sleep.
 
The first, a 1994 study by N. Kurumatani, et al, that appeared in the journal Ergonomics, concludes that individuals need at least 16 hours of time off between shifts to be able to obtain seven or eight hours of sleep.

The second, a 1997 study published in the Chronobiology International, concludes that the time off between the end of one shift and the beginning of the next one should thus be at least 11 hours.

In a separate report examining the limits of consecutive workdays and overtime, Circadian says it generally recommends limiting eight-hour shifts to a maximum of seven in a row, and 12-hour shifts to four or five in a row. 

Sources: Circadian Technologies; Business Week; The Ergonomics Report™, Occupational Health & Safety