A compromise circadian phase position for permanent night work improves mood, fatigue, and performance

Abstract

Study objective: To assess night shift improvements in mood, fatigue, and performance when the misalignment between circadian rhythms and a night shift, day sleep schedule is reduced.

Design: Blocks of simulated night shifts alternated with days off. Experimental subjects had interventions to delay their circadian clocks to partially align with a night shift schedule. Control subjects had no interventions. Subjects were categorized according to the degree of circadian realignment independent of whether they were in the experimental or control groups. Twelve subjects were categorized as not re-entrained, 21 as partially re-entrained, and 6 as completely re-entrained.

Setting: Home sleep and laboratory night shifts.

Participants: Young healthy adults.

Interventions: Experimental subjects had intermittent bright light pulses during night shifts, wore dark sunglasses outside, and had scheduled sleep episodes in darkness.

Measurements and results: A computerized test battery was administered every 2 hours during day and night shifts. After about one week on the night shift schedule, which included a weekend off, the partially and completely re-entrained groups had markedly improved mood, fatigue, and performance compared to the group that was not re-entrained. The completely and partially re-entrained groups were similar to each other and had levels of mood, fatigue, and performance that were close to daytime levels.

Conclusions: Partial re-entrainment to a permanent night shift schedule, which can be produced by feasible, inexpensive interventions, is associated with greatly reduced impairments during night shifts.

Figure 1

Night shifts from 23:00-7:00 (rectangles) are numbered (1-8). Dark horizontal lines in the top panel indicate scheduled sleep episodes for the experimental groups. Sleep episodes for the control groups were self-selected. Arrows show the times of the dim light melatonin onset (DLMO, top panel) and triangles show the estimated sleepiest circadian times (DLMO + 7 h, bottom panel) for individual subjects during the baseline phase assessment (B) and during the final phase assessment for study A (day 32) and study B (day 35). Subjects were assigned to one of 3 re-entrainment categories based on the time of their final DLMO, and was independent of whether they were in an experimental or control group. Dashed vertical lines in the top panel indicate the criteria used to define the 3 re-entrainment groups, while in the bottom panel 7 h was added to these DLMO criteria to facilitate visualization of the sleepiest circadian time. Subjects with a final DLMO earlier than 1:30 were classified as not re-entrained, with a final DLMO between 1:30 and 5:00 as partially re-entrained, and with a final DLMO later than 5:00 as completely re-entrained. Note that the triangles falling in the left-most category (not re-entrained) correspond to the time of the night shifts or commute home. The triangles falling in between the 2 vertical lines (the partially re-entrained category) correspond to times of sleep for experimental subjects on both work days and on days off.

Figure 2

Lapses (reaction time > 500 milliseconds) during the 6:30 test bout of the simple reaction time task. The vertical line between night shifts 3 and 4 indicates that these shifts were separated by 2 days off. Scores are difference-from-baseline. The dotted horizontal lines indicate the baseline mean for all subjects. Symbols denoting statistical significance next to night shift 7 indicate a main effect of group during night shifts 1-7. †† indicates a difference (P < 0.01) between the not re-entrained and completely re-entrained groups. * indicates a difference (P < 0.05) between the not re-entrained and partially re-entrained groups. The tick mark on the right y-axis indicates average baseline scores during the last test bout at the end of the day shifts (16:30). Error bars show SEM. The other ANAM tasks showed a similar progression across night shifts.

Figure 3

Median reaction time (top) and slow responses (bottom) on the procedural reaction time task. Remaining aspects of figure as for Fig 2. Statistical symbols in panels A and C denote the time points at which the groups differed, following a significant group x time-of-night interaction. †(P < 0.05) and ††(P < 0.01) indicate differences between the not re-entrained and completely re-entrained groups. **indicates a difference (P < 0.01) between the not re-entrained and partially re-entrained group.

Figure 4

Median reaction time (top) and slow responses (bottom) on the mathematical processing task. Remaining aspects of figure as for Fig 2. Statistical symbols in panels A, B, and C denote a significant main effect of group. *(P < 0.05) and **(P < 0.01) indicate differences between the not re-entrained and partially re-entrained groups.

Figure 5

Slow responses on the matching to sample and code substitution tasks. Remaining aspects of figure as for Figure 2. Statistical symbol in panel A denotes a significant main effect of group, while symbols in panels B and C denote the time points at which the groups differed, following a significant group x time-of-night interaction. †(P < 0.05) and ††(P < 0.01) indicate differences between the not re-entrained and completely re-entrained groups. *(P < 0.05) and **(P < 0.01) indicate differences between the not re-entrained and partially re-entrained groups.

Figure 6

Mental fatigue (top) and total mood disturbance from the Profile of Mood States (POMS) (bottom) during night shifts. Higher mood disturbance scores indicate greater levels of mood disturbance. Remaining aspects of figure as for Fig 2. Statistical symbols denote significant main effects of group. † indicates a difference (P < 0.05) between the not re-entrained and completely re-entrained groups. *(P < 0.05) and **(P < 0.01) indicate differences between the not re-entrained and partially re-entrained groups.