There's a hormone in your brain called vasopressin, and alcohol switches it off.

Under normal conditions, vasopressin — also called antidiuretic hormone, or ADH — is the signal your brain sends to your kidneys telling them to hold onto water. It's the reason you don't have to urinate constantly. It's a quiet, permanent background process you've never once thought about.

Alcohol interrupts it. Ethanol suppresses ADH release through several mechanisms, including blocking calcium channels in nerve terminals at concentrations as low as 10 mM, and increasing nitric oxide production in the brain regions that control ADH. Without adequate vasopressin, your kidneys stop conserving water and start dumping it.

The effect is fast and the numbers are larger than most people expect. Alcohol can produce urine flow within about 20 minutes of consumption, and this begins even with fairly low-alcohol drinks — beers in the 3–5% range are enough to trigger it. Roughly 250 ml of an alcoholic beverage can prompt the body to expel 800 to 1,000 ml of urine in the hours that follow — about four times more fluid out than in.

That's the mechanism. Every drink you have actively removes more water from your body than it puts in, for hours, while the hormone that would normally stop this sits switched off.

The part most articles get wrong

Dehydration and hangover appear to run on different mechanisms. Dehydration comes from water and electrolyte loss driven by vasopressin and aldosterone suppression. The hangover itself is elicited by oxidative stress and an inflammatory response.

What dehydration does do is stack a second, entirely separate misery on top of the first. Dehydration doesn't cause hangovers on its own, but it makes symptoms considerably worse — low fluid and electrolyte levels magnify the discomfort. The headache, the dry mouth, the flattened, wrung-out fatigue — a meaningful chunk of that is straightforward fluid deficit, and that part is addressable.

So the accurate framing is this: you cannot drink your way out of a hangover, but you can stop yourself from arriving at one already several litres in the red. That's a smaller promise than most articles make. It's also a true one, and it's worth doing.

Why chugging a litre before bed doesn't fix it

The standard move is to drink an enormous glass of water at 2am, right before falling asleep, in the belief that this squares the account.

It largely doesn't, for two reasons.

Your body can only absorb water at a limited rate. Fluid has to pass through the stomach and be absorbed in the small intestine, and there's a ceiling on how fast that happens. Push a litre in all at once and a large fraction of it doesn't get absorbed and used — it sits, then leaves. You feel bloated and nauseated, you get up twice in the night, and you haven't actually banked the fluid.

Your vasopressin is still suppressed. This is the crucial part. Drinking water while ADH is switched off is filling a bucket with a hole in it. The hormone that would normally tell your kidneys to retain that water is offline, so a substantial portion of what you just drank goes straight back out. Your body's baseline hydration level also determines how hard the diuretic effect hits — when you're already mildly dehydrated, alcohol accelerates water excretion faster than when you're well-hydrated, which means falling behind early compounds against you for the rest of the night.

The fluid deficit builds gradually, over hours, drink by drink. It cannot be settled in a single transaction at the end.

What actually works: pacing, not volume

The thing that genuinely helps isn't drinking more water. It's drinking water steadily, throughout, in amounts your body can actually absorb, starting before you're behind.

Dehydration only sets in when diuresis depletes body fluids beyond your intake — which is exactly why alternating water with alcoholic drinks works for maintaining hydration. Roughly a glass of water per alcoholic drink, spaced across the night, keeps intake near enough to output that you never fall deeply into deficit.

And there's a second effect that's arguably more important than the hydration itself, which almost nobody mentions:

Alternating slows down your drinking. A glass of water between rounds occupies the time, the hand, and the ritual that would otherwise be occupied by another drink. Over a four-hour night, that reliably means fewer drinks consumed. And since hangover severity tracks with how much alcohol you actually drank — the acetaldehyde load, the inflammatory response, the sleep disruption — reducing total consumption is the only intervention that touches the real mechanism.

This is the honest answer to "how do I prevent a hangover": drink less. Alternating water is the most socially frictionless way to do that without announcing it or leaving early.

The problem: you will not remember to do this

You know all of the above already, more or less. So does everyone. And it changes nothing, because the plan lives in the version of you that's sober at 6pm, and the execution belongs to the version of you that's three drinks in at 10:30pm in a loud room.

That person is not going to think about vasopressin. That person is not going to voluntarily order a water while everyone else orders another round. And a gentle phone notification is going to be dismissed without being read, or missed entirely because the phone is on silent in a pocket in a venue playing music at 100 decibels.

This is a textbook case for a commitment device: a system you set up in advance, while sober and rational, that constrains your later self at the moment they won't cooperate.

How to set up Sip Siren before a night out

Sip Siren is built for exactly this failure mode. It fires a real alarm — one that cuts through silent mode and Focus — and the alarm doesn't stop until you record a short video proving you actually drank. There's no swiping it away, and no lying to it.

A practical setup:

The video-proof requirement, which sounds ridiculous in the abstract, is the operative part here specifically. At 11pm, a notification you can dismiss is a notification you will dismiss. An alarm that keeps going until you've physically drunk something is one you comply with, because complying is easier than not.