Introduction: Shift work, characterized by irregular schedules resulting in sleep loss and misalignment of circadian rhythms, is associated with increased incidence of cardiovascular disease. We tested whether circadian misalignment has an adverse impact on cardiovascular function independently of sleep loss. Methods: 19 healthy adults were studied using a parallel group design comparing two interventions. Both interventions involved 3 inpatient days with 10-h bedtimes (B1-B3) followed by 8 inpatient days with 5 hours in bed (D4-D10), either with fixed nocturnal bedtimes (circadian alignment n = 8, 3 women, 24.5 +/- 2.7 years old, 23.6 +/- 2.5 kg/m2) or with bedtimes delayed by 8.5 hours on 4 of the 8 days (circadian misalignment, n = 11, 4 women, 22.5 +/- 1.6 years old, 22.1 +/- 2.6 kg/m2). Both interventions were followed by 3 nights of recovery sleep, 2 nights with 12-h bedtimes (R12-13) and 1 night with 10-h bedtimes (R14). During each night, heart rate (HR) and cardiac sympathovagal balance (assessed via the ratio of low frequency to high frequency [LF:HF] in the ECG) were estimated over a 5-min period during stable NREM stage 2, slow wave sleep (SWS) and REM sleep. Only 5-min-periods free from artifacts, arousals, leg movements, breathing instability and ectopic beats were analyzed. A generalized linear model for repeated measures was used to examine between-group differences after adjusting for age and BMI. Results: Total sleep time during the intervention was almost identical in the two groups (4h49min [4 min] vs. 4h46min [6 min]). When compared to the aligned condition, the increase of HR from baseline (B2) was 8 to 10 bpm higher in the misaligned condition during stage 2 (p:0.0009), SWS (p:0.0492) and REM sleep (p:0.0083). LF:HF was higher in the misaligned condition during stage 2 (p:0.0284), SWS (p:0.0450) and REM sleep (p:0.0338). Conclusion: Circadian misalignment as occurs in shift work may impair autonomic cardiac modulation and increase cardiovascular risk, independently of sleep loss.
D. Grimaldi, Sleep Metabolism and Health Center, Department of Medicine, University of Chicago, Chicago, IL, USA