Cortisol-Driven Sodium and Fluid Retention
Stress doesn't just stress you out. It makes you hold water and puff up. Here's the wiring.
Grounded in the research on Cortisol (Wikipedia)
What it actually is
Cortisol is your main glucocorticoid, made in the zona fasciculata of the adrenal cortex and released on the HPA axis (hypothalamus to pituitary to adrenal). Its day job is mobilizing glucose under stress. Its side job nobody warns you about: it makes your kidneys hold onto sodium, and water follows sodium. That's the bloat, the puffy face, the morning that you look softer than you went to bed.
The mechanism: it hijacks the salt receptor
Cortisol can bind the mineralocorticoid receptor (MR) just as well as aldosterone, the actual salt-retention hormone. Your body's fix is an enzyme: 11-beta-HSD2, sitting in kidney tubule cells, converts cortisol to inactive cortisone so it doesn't trip the MR. But that enzyme has a finite capacity. Flood it with cortisol and it gets overwhelmed, cortisol leaks onto the MR, and the kidney starts reabsorbing sodium like it's aldosterone giving the order.