Sistematización de experiencias

Despite the large body of literature attesting to the significant deleterious health effects of sleep disruptions due to shift work there is little research that has systematically investigated the cognitive effects of shift work. Much of the available literature reports on subjective measures of mental fatigue (Tepas et al., 2004) and decreased alertness (Cavallo, Jaskiewicz, & Ris, 2002), and objective measures of reduced work performance, increased errors (Browne, 1949; Hildebrandt, Rohmert, & Rutenfranz, 1974) and increased workplace accidents (Åkerstedt, 1990; Bjerner, Holm, & Swensson, 1955; Folkard & Tucker, 2003; Muecke, 2005). Few report the effects of shift work on cognitive test performance (Rouch, Wild, Ansiau, & Marquié, 2005).

The effects of shift work on memory and speed of processing have been reported by some researchers. Meijman, van der Meer and van Dormolen (1993) compared performance on a delayed match-to-sample memory task in 20 night shift workers on either slow (eight workers: seven day, seven afternoon and seven night shifts) or medium (12 workers: four day, four afternoon and four night shifts) rotating shift systems who had just finished a series of night shifts, with eight control workers on the medium rotating system who had just finished a series of days and afternoons. Baseline measurements were obtained from all workers on the third day off following a period of afternoon shifts. Tasks were administered again to the night workers 32 hours after a series of night shifts, and to the control workers 17 to 25 hours following the last day or afternoon shift. No differences between groups on baseline performance were apparent, but there were significant differences in reaction time and accuracy on the memory task between the night groups and the day group in the recovery phase. Interestingly there were no significant differences in performance between the two different night shift groups, suggesting that duration of night shift is less important in affecting cognitive performance than length of recovery period following the last night shift.

In a large cross-sectional cohort study (N = 3237; males 51.3%) of workers aged 32, 42, 52 and 62 years, Rouch and colleagues (2005) showed that shift

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work experience in males was associated with lower speed of cognitive processing on the WAIS Digit Symbol Substitution test and a letter cancellation task. Longer duration of shift work experience (10 to 20 years vs. 1 to 4 years, adjusted for age and education level) was associated with poorer performance on a verbal measure of immediate recall memory. For both men and women, memory scores were higher for those who had ceased shift work for at least four years compared to those who were currently working shifts. This suggests that the detrimental effects of shift work on cognition are ameliorated with a return to day-only work. However, cohort effects are inherent in such a cross- sectional design. Therefore, longitudinal studies of shift workers’ cognitive abilities would provide more robust evidence for these findings.

Performance on a visual memory span test deteriorated by 18.5% over the course of a night shift in a study examining neuropsychological performance in twelve medical interns who worked four consecutive night shifts (Rollinson et al., 2003). However, as this study failed to report cognitive performance over the course of a day shift as a comparison, it is difficult to discount the effects of general work fatigue, unrelated to shift type, on the interns’ visual memory performance. A study which did make this comparison is that of Smith-Coggins and colleagues (1994), who showed that physicians’ performance on simulated work tasks (intubation of a mannequin and mock patient triage) deteriorated over a night shift as compared to day shift, where performance remained accurate. A study of 16 emergency room residents’ performance on a test of hypothesis testing and decision-making showed that scores were reduced following five consecutive night shifts compared to performance after three day shifts (Dula, Dula, Hamrick, & Wood, 2001).

Electroencephalographic (EEG) correlates of attention, such as the P300 waveform, are affected by rotating night shift work (Yasukouchi, Wada, Urasaki, & Yokota, 1995). P300 is an event related potential (ERP) which provides an electrophysiological measure of attentional resources (Coull, 1998; Muller-Gass & Campbell, 2002) and speed of cognitive processing in humans (Verleger, 1997). This positive waveform appears about 300ms after the detection of a target stimulus, and can be elicited using an auditory oddball task

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(Picton, 1992). Yasukouchi and colleagues (1995) investigated the effects of rotating night shift work and ageing on the P300 ERP by comparing young and older rotating shift workers (14 young female nurses, mean age 24.9 years; 3 older female nurses, mean age 39.0 years; and 12 “elderly” male security guards, mean age 62.8 years) and age-matched controls (young group: n = 12, 50% female, mean age 25.1 years; old group: n = 10, 50% female, mean age 63.4 years). Attentional resources (as measured by P300 amplitude) following a night shift showed a trend towards reduction for the younger nurses and elderly security guards, but were significantly reduced in the older nurse group compared to those measured after a holiday. Speed of mental processing (as measured by P300 latency) following a night shift was significantly reduced only for the elderly security guards. Although these results are somewhat confounded by the low numbers in the older nurse group, the mixture of occupations which perhaps do not involve the same levels of stress, and by the comparison of males and females, they suggest that long-term rotating night shift work (the older nurse group) may lead to a physiologically measurable reduction of attentional resources if sufficient recovery time is not available following a night shift. The increased latency results in the elderly security guard group speak more to the effects of normal ageing on speed of mental processing, rather than to the effects of long-term night shift work.

In contrast to the general finding of cognitive impairment associated with shift work, Lamond and colleagues (2001) found that psychomotor vigilance performance increased in 15 young healthy adults across a week of simulated night work. However, the applicability of this finding to shift workers at large is problematic. It is not clear that long-term night workers, generally a self- selected group who cope well with chronic sleep disruption or shortened sleep duration, would show the same effects as non-shift workers subjected to simulated night shifts.

Adverse effects on mental state, particularly an increase in depressed mood, have been reported following rotating shift work, especially if there is an inadequate recovery period following a block of night shifts. One night of work followed by an inadequate recovery period (straight from night to day shift) has

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been shown to cause significant negative changes in all mood scales of the Profile of Mood States compared to pre-night shift responses (Orton & Gruzelier, 1989).

Significantly higher levels of depressed mood have been reported in medical interns completing night-float rotation compared to day shift (Cavallo et al., 2002). Male shift workers on a rotating shift system including a ‘back to back’ (24-hour) shift and female shift workers rotating between morning and afternoon shifts had higher ‘depression tendency” scores on a self-report measure than did matched day workers in a study by Kaneko and colleagues (2004).

Nurses working on irregular three-shift systems reported higher levels of stress and psychological symptoms such as anxiety, tension and depression than paper mill workers on regular shifts (Olsson, Kandolin, & Kauppinen-Toropainen, 1990). However, given the variety of stressors inherent in the different work environments, it is difficult to attribute this difference to shift work system alone. In contrast, Skipper, Jung and Coffey (1990) did not find a relationship between shift work and measures of depression in their study of 463 nurses, of whom 53.6% had worked rotating shifts for a mean of 48 months.