While research has increasingly investigated stressor–strain and adaptation relationships over time spans of months to years, surprisingly little attention has been focused on shorter mid-term stress reactions. The current research tests the repeated exposure hypothesis which predicts that stressors will produce a mid-term stress reaction; while adaptation occurs over longer timeframes and with repeated exposure. Study 1 adopted a natural experiment combined with intensive longitudinal data collection. Level change in positive affectivity (PA) and negative affectivity (NA) during and after exposure to a novel objective stressor over a 10-day period was modeled using piecewise growth modeling. Consistent with our hypotheses, results identified a decline in PA over time during stressor exposure, and a recovery after stressor cessation. Contrary to expectations, no such effect was identified for NA. Study 2 replicated results of the mid-term stress reaction in Study 1 with a novel stressor and expanded upon Study 1 by comparing the stress reaction of a second group exposed to a familiar stressor. Consistent with our hypothesis, the familiar stressor group demonstrated no stress reaction. Results support the repeated exposure hypothesis while demonstrating that the dynamics of stress reactions over the mid-term can be differential and complex. Theoretical and practical implications are discussed.