Long chain n-3 polyunsaturated fatty acids (n-3 PUFA) derived from oily fish, eicosapentaenoic acid (20:5n-3, EPA) and particularly docosahexaenoic acid (22:6n-3, DHA), can influence cardiac myocyte electrophysiology by modulating ion channels in animal models and in vitro studies [  ]. Prospective cohort studies have found a relatively consistent association between the intake of fish and decreased risk of coronary heart disease death. The GISSI trials showed that an additional intake of 0.85 g n-3 PUFA decreased coronary heart disease mortality, possibly due to an antiarrhythmic action or augmentation of autonomic tone [ 2 , 3 , 4 ]. More recent evidence suggests that n-3 PUFA supplementation has little effect on all-cause mortality, cardiac death, sudden death or myocardial infarction when all trials are considered in totality [  ]. Measuring heart rate variability (HRV) is an indirect, non-invasive approach to assessing cardiac autonomic function [  ]. Low HRV is associated with mortality after a myocardial infarction [ 6 , 7 , 8 , 9 ] and risk of cardiac events in the general population [  ]. A meta-analysis concluded that short-term fish oil supplementation may favorably affect certain frequency domain parameters of HRV [  ]. The present study examines the effects of low doses of n-3 PUFA on HRV, in healthy non-smoking men and women at moderate risk of cardiovascular disease supplemented for 1 y. Here, effects on HRV during sleep-time are investigated, thereby reducing influences from physical activity and emotional/psychological fluctuations.