Objectives:
The purpose of this study was to examine relationship between anhedonia, disruption of biological rhythms, functioning and depression severity in patients with bipolar disorder (BD).

Methods:
The sample included 58 patients with bipolar depression aged 18-65 years. The participants were assessed using the following scales: Dimensional Anhedonia Rating Scale (DARS), Snaith-Hamilton Pleasure Scale (SHAPS), Quick Inventory of Depressive Symptomatology- self-report (QIDS-SR), Biological Rhythms Interview of Assessment in Neuropsychiatry (BRIAN), Functioning Assessment Short Test (FAST). Correlations between the variables were calculated. We built linear regression models with FAST or QIDS-SR as a dependent variable. Mediation analysis was performed.

Results:
Statistically significant correlations were observed between the included variables. Biological rhythms dysregulation and anhedonia were independent predictors of the level of functioning or depression severity. Mediation analysis demonstrated statistically significant mediation of the relationship between anhedonia and depression severity/functioning by the BRIAN score.

Conclusions:
We demonstrated for the first time the interactions between anhedonia, dysregulation of biological rhythms and functioning/depression severity in patients with BD. Reward deficits by causing disruption of rhythm of daily activities and social contacts may result in more difficulties in functioning and higher intensity of depressive symptoms.