The seasonality of depressive symptoms is prevalent in children and adolescents. However, the mechanisms that underlie such susceptibility to seasonal influences on mood disorders are unclear. We examined the effects of a short photoperiod condition on the susceptibility to subchronic unpredictable mild stress (SCUS) and rhythmic alterations of plasma corticosterone (CORT), melatonin, and neuropeptide Y (NPY) in adolescent male rats. Compared with the 12 h/12 h light/dark photoperiod control (CON) rats, the 8 h/16 h photoperiod SCUS rats exhibited significant anhedonia, a core symptom of human depression, together with a blunted diurnal rhythm and elevation of 24 h CORT, melatonin, and NPY levels. The 8 h/16 h photoperiod condition also blunted the rhythmicity of CORT, caused a phase inversion of melatonin, and caused a phase delay of NPY compared with 12 h/12 h CON rats. Such abnormalities of plasma CORT, NPY, and melatonin might cause adolescent individuals to present higher stress reactivity and greater vulnerability to stress over their lifetimes. The present study provides evidence of the susceptibility to the seasonality of stress-related disorders in adolescence.