Some sleepwalkers commit acts of violence, or even murder, in their sleep. Courts must decide what to do with criminal defendants who raise a defense of sleepwalking. A brief review of common law reveals that courts apply the defense inconsistently under various doctrines of justification and excuse. Sleepwalking is a unique medical phenomenon, and courts are poorly equipped to evaluate claims of sleepwalking under existing common law defenses. This Note proposes a single sleepwalking defense based on a balancing test that integrates the medical understanding of sleepwalking.