Attention deficit/hyperactivity disorder (ADHD) can present differently in individuals, with some individuals having difficulty with attentional control, hyperactivity, impulsivity, emotion dysregulation, and/or neurobehavioral functioning. The factors contributing to these different presentations remain unclear, but altered patterns of physical activity, sleep, and circadian rest/activity rhythms may play a key role. The goal of this study is to leverage wearable technology (i.e., a wristband) to investigate the relationships between physical activity during the day, sleep patterns and disturbances, and 24-hour circadian rest/activity rhythms with differences in ADHD symptoms, emotion dysregulation, and related brain and behavioral features of attention-deficit/hyperactivity disorder (ADHD). The investigators hope this study will help improve assessment and intervention for individuals with ADHD by understanding how these factors relate to ADHD symptom expression and associated brain differences in ADHD. Participants taking stimulant medication must withhold stimulant medication 24 hours before their research appointment and the morning of their research appointment. Stimulant medication may be restarted after the appointment is complete. Participation in this study will require children to complete an initial 2-hour research appointment, two (2) weeks of activity and sleep monitoring at home using a wearable wristband and answering questions sent to a smartphone, and a second 4-hour research appointment after the 2-week period. During the first research appointment, children will complete a cognitive assessment and a practice magnetic resonance imaging (MRI) scan. Parents/legal guardians will participate in the 30-45-minute sleep device training session with one of the research staff. During the two weeks of activity/sleep monitoring at home, parents and children will answer questions about their sleep routine, ADHD symptoms, and emotional responding each morning and evening. Parents will be asked to install a questionnaire application on their smartphone. A prompt will be sent to their smartphone multiple times per day reminding parents to complete the brief assessment. After the 2-week period, children will complete a 4-hour research appointment. During this research appointment, children will complete a 60-minute MRI scan and computer-based activities that assess cognitive skills, reward-based decision-making, and frustration tolerance. At the end of the research appointment, children will return the device to our research team. Parents may delete the questionnaire application from their phone at the end of the research appointment. Participation will also require parents/legal guardians to complete questionnaires about their child. Questionnaires will be provided to the primary caregiver by email or at the beginning of their child's first research appointment. Parents agree to complete and return the questionnaires within one month of their child's research appointment. Parents may be provided with additional questionnaires to give to their child's primary schoolteacher. This information is collected to better understand children's abilities, behavior, strengths, and weaknesses. There are minimal risks associated with this study. Risks include fatigue, boredom, and mild discomfort. There is no cost to participating in this study. There is no direct benefit to participants for participating in this study.