Increased vagal tone during winter in subsyndromal seasonal affective disorder

Background: Seasonal affective disorder (SAD) is characterized by recurrent winter depression with summer remissions and/or hypomania. Further symptoms include hypersomnia, increased appetite, weight gain, fatigue, and social withdrawal, which may indicate autonomic changes during winter. Methods: Measurements of respiratory sinus arrhythmia, heart rate (HR), and skin conductance level (SCL) were taken from 32 participants in subsyndromal SAD and control groups (eight male and eight female subjects in each group) in autumn and winter to determine any change in autonomic function. Measures were taken at baseline and during two stressor tasks. Single determinations of blood pressure, sublingual temperature, depression, aerobic fitness, and body mass index were also measured at each session. Replication in a second data collection period over subsequent winter and spring periods was conducted with an additional 32 participants to extend the findings and to counterbalance order effects in testing. Data were combined to produce “winter” and “nonwinter” test periods and statistically corrected for testing order. Results: Respiratory sinus arrhythmia differences indicated that SAD subjects have increased vagal tone in winter. Both groups show a decrease for HR and increases for SCL and diastolic blood pressure in winter. Conclusions: Seasonal affective disorder may show similarities with hibernation, and the results may indicate mechanisms different from those of nonseasonal depression.