Inattentiveness and impulsivity in healthy adults and individuals with attention deficit hyperactivity disorder (ADHD) diminish these individuals' quality of life. Cortical hypometabolism has been hypothesized to contribute to inattentiveness in ADHD. Transcranial photobiomodulation (tPBM) is a safe and non-invasive modality for activation of the prefrontal cortex. tPBM in healthy adults has been shown to augment brain oxygenation, cognition, and mood. The cognition-enhancing effect of tPBM on sustained attention in adults with ADHD remains unstudied. The investigators will evaluate whether tPBM administration to adults with and without ADHD can improve their attention. Each participant will be screened for eighteen criteria based on the Diagnostic and Statistical Manual (DSM-IV, text revised), using the validated instrument Adult ADHD Self-Report Scale (ASRS-v1.1). Blinded participants will be randomized into (I) Active-tPBM or (II) Sham-tPBM groups. After undergoing an 8-minute administration of Sham or 1064 nm tPBM to the lateral and medial right prefrontal cortex, each participant will complete a 14-minute, computer-based Conner's continuous performance task-3 (CPT-3). The investigators will examine whether adults with and without ADHD who receive Active-tPBM improve in measures of inattention, impulsivity, sustained attention, or vigilance compared to the Sham-tPBM group. Cognitive assessments of attention like the CPT-3 may evaluate the efficacy of tPBM to increase cortical metabolism in healthy adults and individuals with conditions like ADHD. The investigators anticipate their investigation will be a starting point for more sophisticated studies that focus on the implications of tPBM on cognition.