How does alcohol affect hydration?

Alcohol inhibits antidiuretic hormone (ADH/vasopressin) secretion, increasing urine output by 3–4 times the volume of alcohol consumed, meaning a standard drink (14 g ethanol) causes a net fluid loss of approximately 100–200 ml, producing the dehydration responsible for next-day hangover symptoms including headache, raised resting heart rate, reduced HRV, and cognitive impairment (Hobson & Maughan, Eur J Sport Sci, 2010).

This mechanism is dose-dependent: at very low alcohol doses (one standard drink), ADH suppression is partial and the net fluid impact is small. At moderate to heavy drinking (3+ standard drinks), the cumulative fluid loss is sufficient to cause clinically meaningful dehydration by morning. Beer has the lowest dehydrating effect among alcoholic beverages because of its high water content (a 330 ml beer is approximately net-neutral for fluid balance at 4–5% ABV); spirits and wine are more dehydrating relative to their volume.

Honesty Scale: Solid (1) for alcohol's ADH suppression and dehydrating mechanism.

What to do: For every standard alcoholic drink, consume approximately 200–250 ml additional water. This partially compensates the ADH suppression effect. Consuming water before bed after drinking reduces (but does not eliminate) morning dehydration-related symptoms and the heart rate and HRV effects that your wearable will report.

For the full picture, read The Hydration Deep Dive