Assessing the Impact of Consecutive Night Shifts on Night-time Alertness, Daytime Sleep and Timing of the Circadian System

In the Australian coal mining industry, most guidelines for managing the risks associated with human fatigue recommend a maximum limit of four consecutive 12-hour night shifts. This approach is consistent with the prevailing view of experts in the fields of sleep and circadian rhythms. However, there is some evidence to suggest that longer blocks of night shifts may be preferable if ambient lighting conditions facilitate adaptation of the body clock. The aim of this project was to assess body clock alignment, sleep and cognitive function during seven consecutive night shifts with lighting conditions similar to those in open cut and underground mines.

24 people from the general community (7F, 17M, aged 36.2 ± 9.0 years) volunteered for a 9-day shift work simulation study. Participants were randomly assigned to one of two conditions: (a) Night Shift (n=13) - 7 x 12-hour night shifts with 8 hours in bed each day, and (b) Day Shift (n=11) - 7 x 12-hour day shifts with 8 hours in bed each night. The day shift condition was included as a control to check that any effects observed in the night shift condition were not due to living in an accommodation suite at a sleep research facility. Light intensity was low during night shifts (<10 lux), except for 2 x 30-minute periods of moderate-intensity light (~350 lux) in the fifth and ninth hours of the shifts. Light intensity was moderate during day shifts (~350 lux).

For participants in the night shift condition, there were minor improvements in body clock alignment, alertness and sleepiness, and minor impairments in sustained attention and response speed, over the week - but none of these changes were statistically significant. Similarly, there were no significant changes in the sleep of these participants over the week. For participants in the day shift condition, there was no indication that living in an accommodation suite had any substantial effects on body clock alignment, quantity/quality of sleep or cognitive function.

The lack of substantial adaptation of the body clock during the week of night work in lighting conditions similar to those in open cut and underground mines provides support for the argument that blocks of night shifts should be shorter rather than longer. In contrast however, sleep, sustained attention, response speed, alertness and sleepiness were all maintained throughout the week of night work, which indicates that schedules with up to seven consecutive night shifts may be reasonable - particularly if an optimal sleeping environment is available. A potential benefit of working longer, but fewer, sequences of night shifts, is that it reduces exposure to the risks associated with 'first night effects' and with long commutes to/from remote sites.