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 symptom s 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 front 32 participants in subsyndro mal SAD and central groups (eight male and eight female subjects in Each gr oup) in autumn and winter to determine any change in autonomic function. Me asures were taken at baseline and during two stressor tasks. Single determi nations of blood pressure, sublingual temperature, depression, aerobic fitn ess and body mass index were also measured at each session. Replication in a second data collection period over subsequent winter and spring periods w as 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 testin g order. Results: Respiratory sinus arrhythmia differences indicated that SAD subjec ts 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 hiberna tion, and the results may indicate mechanisms different from those of nonse asonal depression. Biol Psychiatry 2001;50:28-34 (C) 2001 Society of Biolog ical Psychiatry.