Why do you get sick right after a stressful period ends?

Falling ill the first day of vacation is common enough to have been studied. During sustained stress, cortisol and catecholamines modulate immune function, and while acute stress can actually enhance some defenses, chronic stress suppresses aspects of the adaptive immune response and shifts the body toward keeping you operational rather than fighting infection. When the stressor ends and those hormone levels fall, immune activity rebounds, and since many symptoms of illness, the aches, congestion, and fever, are produced by the immune response rather than by the pathogen directly, the symptoms arrive as the system reactivates. You may have been infected during the stressful period and only felt it afterward.

Behavior compounds the physiology. Stressful stretches typically involve short sleep, poor eating, skipped exercise, and more exposure to other people, all of which raise susceptibility, and the immediate aftermath often adds travel and time zone changes. Research on migraine has documented a parallel let-down pattern, with attack risk elevated in the period immediately following stress rather than during it. The practical implication is that recovery is not automatic: protecting sleep during the demanding period rather than only afterward, and tapering rather than dropping straight from maximum load into nothing, appear to blunt the effect. This is also a reasonable argument for taking rest deliberately and regularly rather than banking it for a single collapse at the end.