It would seem from this research that 12-hour shifts are no more hazardous than 8-hour shifts. For the workers studied here it would seem that the 12-hour shift was preferred.
MD Johnson et al. International Journal of Industrial Ergonomics (2001) Vol.27 p.303.
Many industries and services have begun to introduce rotating 12-h schedules. There have been concerns that such schedules may influence the rate of occupational injuries, workers health, and factors that may contribute to stress. Established 8-hour shift patterns are already known to produce sleep disturbance.
412 employees were selected for the study, which was controlled and selected on the basis of accurate representation of work types and employment contracts. Response rates were over 80%. Results from before and after the change in shift pattern were compared.
With respect to amount of sleep, the responses were significantly in favour of the 12-h schedule over the 8-h schedule.
The study also indicated that there was no significant change in the occupational injury rate or its effect on sickness absence.
Subjective feelings about work and work/life balance also favoured the 12-hour shift.
Given that none of the outcome measures were truly objective or precise, it would seem from this research that 12-hour shifts are no more hazardous than 8-hour shifts. For the workers studied here it would seem that the 12-hour shift was preferred.