This study investigated whether early signs of attention deficit hyperactivity disorder (ADHD) in toddlers aged 2–3 years are associated with disturbed sleep and activity levels. Participants were recruited from the Odense Child Cohort, and children scoring above the 93 rd percentile on the ADHD scale of the Child Behaviour Checklist 1½–5 were categorised as cases and compared with age- and gender-matched normal-scoring controls. Daytime and nocturnal activity for 24 children with ADHD traits (cases) and 25 healthy controls was assessed through 7 days of actigraphy, and parents completed the Children's Sleep Habits Questionnaire (CSHQ) and the ADHD Rating Scale IV Preschool Version (ADHD-RS). Cases differed significantly on actigraphic parameters by having fewer minutes of moderate-to-vigorous physical activity (MVPA), prolonged total sleep time, fewer sleep interruptions, and increased night-to-night variability. A significant association was found between fewer minutes of MVPA and higher parent-reported motor activity on the ADHD-RS. Furthermore, increased night-to-night variability was significantly associated with higher total scores on both CSHQ and ADHD-RS. The findings show that early signs of ADHD are associated with an irregular sleep pattern and lower daytime activity, as illustrated by actigraphy. Studies investigating early ADHD risk factors could lead to a preschool ADHD risk index to help guide future early intervention.