Objective This study examined the association between sleep habits and problems and Internet addiction in adolescents. Methods Junior high school students from a local town in Japan (n=853; male/female, 425/428) were the subjects of this study, and were assessed for severity of Internet addiction and sleep habits and problems using the self-reported version of the Young’s Internet Addiction Test (IAT) and Child and Adolescent Sleep Checklist (CASC). Results The wake time on weekdays was not significantly different among the three groups; addicted, possibly-addicted, and non-addicted. In the addicted group, the total night sleep time was significantly shorter, and the bedtime was significantly delayed on both weekdays and weekends compared with those in the possibly-addicted and non-addicted groups. The wake time of the addicted group was significantly later than that of the other groups. The total scores of sleep problems measured by the CASC were significantly higher in the addicted and possibly-addicted groups than in the non-addicted group. Conclusion Internet addiction is strongly associated with sleep habits and problems in adolescents. These findings suggest that internet addiction should be considered while examining adolescent lifestyle.