Background: Rating scales used to assess the severity of depression e.g. the Hamilton Depression Rating Scale 17-item (HDRS-17) partly rely on the patient’s subjective experience and reporting. Such subjective measures tend to have low reliability and adding objective measures to complement the assessment of depression severity would be a major step forward. Aims: To investigate correlations between electronic monitoring of psychomotor activity and severity of depression according to HDRS-17. Methods: A total of 36 patients with unipolar disorder (n = 18) or bipolar disorder (n = 18) and 31 healthy control persons aged 18–60 years were included. Psychomotor activity was measured using a combined heart rate and movement sensor device (Actiheart) for 3 consecutive days, 24 h a day. Results: We found that sleeping heart rate (beats/min) correlated with HDRS-17 in both patients with unipolar disorder and bipolar disorder (unadjusted model: B = 0.46, 95% CI 0.037–0.89, P = 0.034). In contrast, correlations between activity energy expenditure (kJ/kg/day), cardio-respiratory fitness (mlO2/min/kg) and HDRS-17 were non-significant. Conclusions: These results suggest that measuring sleeping heart rate in non-experimental daily life could be an objective supplementary method to measure the severity of depression and perhaps indicate presence of insomnia.