Little is known regarding sleep and baseline concussion performance and symptoms in athletes. We explored the effects of self-reported prior night’s sleep quantity on baseline symptoms and computerized neurocognitive testing in high school athletes. A retrospective analysis of 958 high school athletes between the ages of 13-19 years was completed on Immediate Post-Concussion Assessment and Cognitive Testing (ImPACT) performance. Participants were categorized into two groups based on self-reported prior night’s sleep quantity: <8 h (n = 524; 55%) and >/=8 h (n = 434; 45%). Measures consisted of baseline total symptom score, symptom factors (vestibular-somatic, sleep-arousal, affective, and cognitive-sensory), and ImPACT composite scores (verbal and visual memory, visual-motor speed, reaction time, and impulse control). Significant differences were found on all baseline symptom factors (ps < .02) and total symptom scores (p < .001) with greater symptom reporting in the <8 h prior night's sleep group. No group differences were observed between duration and composite scores of verbal memory (p = .49), visual memory (p = .94), visual-motor speed (p = .38), reaction time (p = .50), or impulse control (p = .81). High school athletes who report <8 h of sleep the night prior to baseline concussion testing reported greater symptoms across all symptom factors. Clinicians should consider inadequate sleep the night prior to concussion testing to ensure accurate, valid assessments, especially on symptom reporting scales.