Is diurnal variation of mood associated with parasympathetic activity?

18 patients with distinct improvement of mood in the evening and 18 patients without, both suffering from major depression and equally treated with tricyclic antidepressants (TCA), and an age- and sex-matched group of 18 normal control subjects underwent a standardized heart-rate (HR) analysis (HRA) in the morning (08:00) and 12 h later in the evening (20:00). The battery of cardiovascular reflex tests included the determination of HR variability (HRV) while resting and during deep breathing, and a spectral HRA. The depressed patients with diurnal variation of mood showed significantly decreased HR and significantly increased HRV parameters while resting and during deep respiration in the evening. On the contrary, patients without diurnal changes of mood just showed a significant HRV increase during deep respiration in the evening. No statistically significant changes of these parameters were detected in the healthy subjects. It is not known if the observed changes of HR parameters representing increment of parasympathetic tone are intrinsically related to the mood swings or if this rhythm just becomes visible under the circumstances of depression. Also, the impact of TCA treatment upon the results remains to be elucidated.