Background: Circadian disturbances may play a key role in the pathogenesis of some forms of mood disorders. Despite marked changes in circadian rhythms during the normal course of adolescence and young adulthood, less is known about changes in the 24-h sleep–wake cycle in young persons with mood disorders. Methods: Seventy-five young participants with mood disorders (unipolar: n=46, 20.1 ± 4.7years old; bipolar I or II: n=29, 23.2 ± 4.3) and 20 healthy participants (24.8 ± 2.5 years old) underwent actigraphy monitoring during a depressive phase over seven consecutive days and nights. Sleep phase delay was defined as mean sleep onset ≥1:30am and/or sleep offset ≥10:00 am. Results: A delayed sleep phase was found in 62% of participants with bipolar disorders when depressed, compared with 30% of those with unipolar depression (χ2=6.0, p=0.014) and 10% of control participants (χ2=11.2, p=0.001). Sleep offset times were significantly later in subjects with mood disorders compared to the control group, and later in those with bipolar as compared with unipolar disorders (all p≤0.043). Limitations: This study was cross-sectional and the depressed groups were somewhat younger compared to the healthy controls. Longitudinal studies are required to determine the predictive significance of these findings. Conclusions: Young patients with mood disorders, especially those with bipolar disorders, are particularly likely to have a delayed sleep phase. Therapies focused on advancing sleep phase may be of specific benefit to these young persons.