We emphasize the potential importance of the role of early alterations in sleep and circadian rhythms as a biological marker of early-onset depression in the preschool period. This builds on findings of the reciprocal relationship between sleep and mood as well as the validity of preschool depression well established in the extant literature. This editorial highlights two recent studies published in JCPP in 2024 defining the duration of clinically impairing depressive symptoms in young children and methods that are now feasible to track daily patterns of sleep and circadian rhythms and show their relation to mood. We propose future studies to investigate these relationships in young children at risk for depression.