Dynamics of the human stress system in depression: A combined population- and person-based approach to assess long-term changes and daily life fluctuations

Depression is a stress-related disorder, with an often chronic course. Studies into the biology of depression have often focused on a major component of the stress system, the hypothalamic-pituitary-adrenal (HPA) axis, which increases the release of the hormone cortisol upon activation by stress.

Studies into the amount of cortisol in depressed versus non-depressed samples show inconsistent results. A possible reason for this is that they did not account for the fact that the production of cortisol fluctuates over the day and that functioning of the HPA axis may change over time. Studies described in this thesis suggest that the cortisol stress response is increased in individuals with acute depressive problems, but that it is decreased in individuals with a longer history of depressive problems. In addition, they also suggest that the presence of a relationship between depression and increased cortisol levels at the group level does not imply that depressed individuals can be discriminated by their cortisol levels. Therefore, the use of cortisol as biomarker for depression is currently ruled out.

In this thesis, it was also examined whether a possible antidepressant effect of physical activity on depressive symptoms is explained by changes in functioning of the HPA axis. Regular exercise appeared to decrease depressive symptoms as expected, but changes in the cortisol stress response did not seem to underlie this effect. In addition, it was found that daily physical activity leads to an increase in positive emotions in nearly everyone, while the effect on negative emotions differs between individuals.