To test the hypothesis that electrooculogram contains information on autonomic functions, correlation analyses of electrooculogram and heart rate variability (HRV) parameters during night sleep and over 24 h were performed on 24 healthy young volunteers (12 women and 12 men). Continuous frequency-domain analysis revealed repeated emergence of electrooculogram low-frequency power (PEOG, 0.05-0.5 Hz) during night sleep. The change in the PEOG, when natural log transformed, was graded rather than all or nothing. The PEOG was not correlated with high-frequency power (HF, 0.15-0.4 Hz) of HRV. In contrast, the PEOG was significantly correlated with R-R interval (r=-0.46+/-0.15; mean+/-SD, P<0.05) and low-frequency power (0.04-0.15 Hz) to HF ratio (LF/HF) of HRV (r=0.49+/-0.10, P<0.05). The correlation coefficient between PEOG and R-R interval and between PEOG and LF/HF became even larger (r=-0.68+/-0.08 and 0.58+/-0.09, respectively) when 24-h recordings were analyzed. There was no significant difference in the correlation between women and men. We concluded that the electrooculogram, as analyzed in the frequency domain, contains information on sympathetic activity not only during night sleep but also throughout day and night in healthy young people.