Background: The Internet has been gaining worldwide popularity in recent years, but a loss of control over Internet use might lead to negative impacts on our daily lives., Objectives: This study explored the effects of Internet addiction on autonomic nervous system function through heart rate variability (HRV) analysis., Methods: This was a cross-sectional design. Data were collected from 240 school-aged children who completed the Chinese Internet Addiction Scale and Pittsburgh Sleep Quality Index questionnaires. Spectral analysis was used to measure HRV. Independent t test was used to compare differences in characteristics and HRV between groups. A 2-way analysis of variance was used to examine group differences in HRV., Results: Internet addicts had significantly lower high frequency (HF) percentage, logarithmically transformed HF, and logarithmically transformed total power and significantly higher low frequency percentage than did nonaddicts. Internet addicts who had insomnia had higher low frequency percentage and lower HF percentage, logarithmically transformed HF, and logarithmically transformed total power compared with nonaddicts who did not have insomnia., Conclusions: Internet addiction is associated with higher sympathetic activity and lower parasympathetic activity. The autonomic dysregulation associated with Internet addiction might partly result from insomnia, but the mechanism still needs to be further studied.