Journal of Electrocardiology Vol. 30 No. 1 1997 Circadian Heart Rate Variability Rhythm in Shift Workers Jofio Freitas, MD, Pedro Lago, PhD, Jorge Puig, MD, Mfirio J. Carvalho, MD, Ovfdio Costa, PhD, and A. Falcfio de Freitas, PhD Abstract: The objective of this study was to assess the influence of day-night cycle and sleep-awake period on the circadian pattern of heart rate variability (HRV). Twelve male oil refinery security shift workers, aged 39 _+ 7 years, 'were studied with 24-hour Holter monitor recordings during morning and :night work periods. Hourly HRV parameters in the time and frequency domains were evaluated. For both shifts, all HRV parameters during awake or work periods were found not to be statistically different. In both day and night work shifts, the very low frequency and high-frequency components of HRV and the proportion of differences in successive R-R intervals greater than 50 ms increased during the sleep period, while the low frequency/high fre- quency ratio decreased. The low-frequency component in absolute units and the SD of the R-R interval did not show any variation in either shifts for the different periods. These results suggest that the circadian pattern of HRV seems to be predominantly related to sleep (supine) and wakefulness (stand- ing) and remains independent of night-day cycle. Key words: circadian rhythm, heart rate variability, shift workers, autonomic nervous system. Previous studies indicate that the circadian tim- ing system of the average night shift worker failed to adapt successfully to such work schedules. This could lead to a misalignment between the desired work/rest schedule and the output of the intrinsic circadian pacemakers and to disturbed daytime sleep (1). Consequently, sleep deprivation could produce decreased alertness and performance in night shift workers. In addition, shift workers are believed to have higher rates of cardiac, gastroin- testinal, and reproductive disorders (1). From the Centro de Medicina Desportiva do Norte, Centro de Estu- dos da Funf~o Auton6mica Corino Andrade, and Departamento de Matematica Aplicada, Oporto University, Oporto, Portugal. Reprint requests: Jo~o Freitas, MD, Centro de Medicina Des- portiva do Norte, Rua Antdnio Pinto Machado, 32, 4100 Porto, Portugal. Some key cardiovascular events depend at least partly on the autonomic nervous system and have a clear circadian rhythm, such as sudden death, ectopic ventricular activity, Q wave myocardial infarction, stroke, episodes of angina pectoris, bursts of hypertension, and platelet aggregability (2-5). In normal populations, autonomic activity also shows a circadian pattern, which is similar to that of cardiovascular events, being probably one of the link effects. The sympathetic activity is prevalent during the day and in the early waking hours, and the parasympathetic component is predominant at night (6). Heart rate variability (HRV) in the time (7) and frequency domain (8) seems to be a powerful and a reproducible tool to assess autonomic balance. The aim of the this study was to assess the influence of the day and night shift periods of oil refinery work- 39