Attention deficit/hyperactivity disorder (ADHD) is a childhood-onset psychiatric condition characterized by hyperactivity, impulsivity, and attention deficit. In addition to these core symptoms, accumulating evidence suggests that ADHD may also involve alterations in circadian rhythms, sleep disturbance, and time perception. The habenula is a brain region transmitting limbic information to the midbrain monoamine systems and is thereby involved in the regulation of monoamine release in such target brain areas as the striatum, where it is part of the biological substrates that process time perception. The habenula is additionally a part of the circadian rhythm network and is involved in sleep regulation. Our recent study provides a new insight that a habenula lesion created early in development produces behavioral and brain alterations resembling those observed in ADHD. We propose that the habenula may be a promising target for understanding the pathogenesis of ADHD.