The things that used to roll off you don't anymore. You are not weaker. Your stress regulation system has been neurobiologically reconfigured by perimenopause. The Swiss Perimenopause Study confirms cortisol rises during the transition — at the exact moment the sex hormones that buffer and modulate it decline. The HPA axis fires at lower thresholds. Recovery takes longer. The overnight cortisol nadir is disrupted. Here is the science of what changed — and the specific toolkit