Introduction: We examine whether insomnia and excessive sleepiness, the two diagnostic symptoms of Shift Work Disorder (SWD), are differently related to evoked responses and work impairment. Methods: 34 night workers participated in an overnight MSLT and evoked potential assessment. Subjects had no sleep disorders prior to starting night work. At 17:00, each subject completed an Endicott Work Productivity Scale (EWPS), two Insomnia Severity Indices (ISI-Day, ISI-Night), and an Epworth Sleepiness Scale (ESS). Subjects with ISI-Day >/= 10 and ESS < 10 were classified "alert insomniacs" (AI, n = 12). Subjects with ISI-Day >/= 10 and ESS >/= 10 were classified "sleepy insomniacs" (SI, n = 11). Subjects reporting < 10 on both scales were classified controls (n = 11). At 18:00, subjects completed a test of attention to novelty and associated ERPs. Results: Neither the MSLT nor the ESS correlated with EWPS scores or ERP amplitudes (p > .10). However, the mean of the ISI measures correlated with the EWPS (r = .409, p < .01) and the attention-to-novelty P3a (r = -.410, p < .01). The AI group was most impaired on the EWPS, significantly more impaired than controls (25.8 +/- 14.8 vs. 12.3 +/- 9.4, p < .05). SI were not statistically different from controls (19.5 +/- 8.7 vs. 12.3+/- 9.4, p > .05). Interestingly, the fatigue subscale of the EWPS was significantly higher in AI than in controls (6.3 +/- 3.1 vs. 3.4 +/- 2.5, p < .05), while there was no significant difference between SI and controls (4.8 +/- 1.7 vs. 3.4 +/- 3.1, p > .10). Compared to controls, AI showed significantly attenuated P3a responses (Fcz, Czp, Cpz, MD 1.62-1.77, p < .05) and target-detection P3b responses (Fcz, Czp, Cpz, MD 1.28-1.64, p < .05). P3b in SI was not different from controls (p > .10) and P3a was only different at one electrode (Cpz, MD 1.43, p < .01). Conclusion: Insomnia is linked to functional and cognitive impairments in shift workers. Insomniacs with normal sleepiness showed more severe impairments than insomniacs who reported excessive sleepiness.
R. Belcher, Sleep Disorders and Research Center, Henry Ford Hospital, Detroit, MI, USA