If you wear a recovery tracker and drink, you have already seen this. A normal evening out produces a morning score that looks like you ran a hard session, and the reason it surprises people is that the effect outlasts the alcohol by a considerable margin.
What actually happens overnight
Alcohol is sedating, so it shortens the time it takes to fall asleep, which is why it has a persistent reputation as a sleep aid. That is the only part of the night it helps.
As your body metabolises it through the night, the sedative effect reverses into a rebound. The second half of the night becomes lighter and more fragmented, with more waking, more time in light sleep and suppressed REM. This is also why waking around 3am after drinking is close to a textbook pattern, covered in why do I wake up at 3am.
Meanwhile your heart rate stays elevated and HRV stays suppressed through the whole night, because your autonomic nervous system is handling the metabolic work rather than downshifting the way it should during sleep.
Why it reaches into the second night
This is the part people find counterintuitive. The alcohol is long gone by the following evening, yet the numbers often have not fully returned.
Two reasons. First, you are carrying a sleep debt from a fragmented night, and short or poor sleep suppresses HRV on its own. Second, recovery from the disruption is not instantaneous: sleep architecture tends to rebound over subsequent nights rather than snapping back immediately.
The practical consequence for anyone training: a Friday night out is not a Saturday problem, it is a Saturday and Sunday problem.
What reduces the damage
Drink earlier. The highest-leverage change. The more of the metabolism that happens while you are awake, the less of it interferes with sleep. Finishing three or four hours before bed makes a visible difference to the same total.
Drink less. Obvious, and the effect is genuinely dose-dependent, so reducing from four drinks to two is worth real points rather than being all or nothing.
Hydrate alongside. Modest help. Dehydration independently raises heart rate and lowers HRV, so removing it removes one of the compounding factors.
Eat with it. Slows absorption, which flattens the peak. Though a very large meal close to bed brings its own overnight cost.
What does not work: a hard workout the next morning to sweat it out, which mostly adds load on a day when your capacity to absorb it is reduced.
Should you train the next day?
Easy work, generally yes. Hard intervals or heavy lifting, generally no, for two reasons. Your capacity is genuinely reduced, and the adaptation you would be training for is blunted by poor sleep anyway. You take the cost of the session and receive less of the benefit.
This is one of the cases where a recovery score is doing something genuinely useful: it is not telling you something mystical, it is telling you what you already suspected with enough specificity that it is harder to argue with.
For the other substance with a large overnight effect, see how long caffeine actually affects your sleep, and for the full ranking of causes see why is my HRV low.