SYNTH-77 · Guide

Alcohol and training

One or two drinks cost REM sleep and next-morning HRV and nothing else that has been shown; a heavy night after damaging work is the case to plan around

Blimited evidenceMostly from a mixed groupWritten before October 2026, being re-checked

Decision

Dose decides: one or two drinks cost REM sleep and next-morning HRV only — train as planned and read the morning numbers as alcohol; after a heavy night (about 1 g/kg or more) that follows damaging work, move the next key session at least a day later; at any dose the recovery carbohydrate is eaten first.
In detail:

1. One or two drinks (up to about 0.5 g/kg): REM sleep is already reduced REF-1010 and overnight HRV recovery already lower REF-1011. No evidence of harm to muscle repair or glycogen at this dose — none was tested. Train as planned; read the next morning's HRV and resting heart rate as alcohol, not as fatigue.

2. A heavy night (about 1 g/kg and above, five or more drinks): the REM loss is larger REF-1010; after damaging work — a race, a long descent, a first heavy leg session — strength loss over the next 36–60 hours can be about twice as deep REF-1014; at 1.5 g/kg muscle protein synthesis after training is about a quarter lower even with protein REF-1012. Move the next key session at least a day later rather than testing it.

3. Whatever the dose: eat the recovery carbohydrate first. Glycogen suffers when alcohol replaces the meal, and was restored by 24 hours when the carbohydrate was eaten REF-1013.

Before an A race or a threshold or VO2max test: the only lever the evidence gives is dose, and for sleep there is no threshold above zero. State the trade and let the athlete choose.

Evidence

Sleep: meta-analysis of 27 studies, REM disruption from ≤0.50 g/kg, worsening with dose REF-1010.
Overnight HRV: 4098 people, recovery index down 9.3 / 24.0 / 39.2 percentage units at low / moderate / high intake, not blunted by fitness REF-1011.
Muscle protein synthesis: −24% at 1.5 g/kg with protein, eight men REF-1012.
Glycogen: reduced when alcohol displaced carbohydrate; direct effect unclear REF-1013.
Force after eccentric damage: deeper loss at 1 g/kg, 11 men REF-1014.

Practical Application

Answer with the dose first: one or two drinks cost sleep quality and next-morning HRV, and nothing else that has been shown; a heavy night after hard eccentric work is the case to plan around.
Never present "alcohol ruins tomorrow's endurance session" as a finding: no study in this set measured next-day endurance performance.

Limitations

Muscle and glycogen evidence exists only at 1–1.5 g/kg in small all-male samples; anything said about one or two drinks for those outcomes is extrapolation and must be labelled so.
Sleep and HRV studies are in the general population, not trained endurance athletes; almost no data in women for muscle, glycogen or force outcomes.
This is not guidance on alcohol and health; an athlete describing dependence or drinking to cope is referred to a doctor.