Alcohol is one of the topics in nutrition where the gap between what people know intellectually and what they account for in practice is widest. Most people who drink are aware that alcohol has some health implications. Fewer have a clear picture of specifically what it affects and through which mechanisms. A plain account is more useful than a moral one.
What alcohol does to sleep
Alcohol is sedating in the short term, which is why it is associated with falling asleep more easily. The problem is what happens during the night.
As the body metabolises alcohol, the process produces acetaldehyde — a toxic byproduct that acts as a stimulant as blood alcohol concentration falls. This creates fragmented sleep in the second half of the night: more light sleep, less REM sleep, more waking. REM sleep is the sleep phase most associated with cognitive recovery, emotional regulation, and memory consolidation.
Even moderate alcohol consumption — 1–2 drinks — can reduce REM sleep by 20–30% in the portion of the night when alcohol is being metabolised. A person who sleeps for 8 hours after drinking may wake less rested than someone who slept for 6 hours without drinking, because the quality of sleep is substantially different.
For recovery from training, sleep is the primary recovery mechanism. Growth hormone — critical for muscle repair — is released predominantly during deep sleep. A night with disrupted sleep architecture is a less effective recovery night, regardless of how many hours were spent in bed.
What alcohol does to muscle protein synthesis
Research published in 2014 by Parr and colleagues found that alcohol consumption after exercise significantly reduced muscle protein synthesis, even when protein was consumed alongside the alcohol. A dose of approximately 7 drinks after a combined strength and endurance session reduced MPS by around 37%. When protein was co-ingested, MPS was still reduced by approximately 24%.
The mechanism involves alcohol's inhibition of mTOR signalling — the intracellular pathway that regulates protein synthesis in response to exercise and amino acid availability. Alcohol competes with the recovery stimulus at a molecular level.
It's worth contextualising this: the dose in the Parr study was significant (approximately 7 standard drinks), and the session was demanding. Smaller amounts of alcohol after modest training produce a smaller effect. But the directionality of the relationship is consistent across the research: alcohol after training reduces recovery quality, with dose-dependent magnitude.
What alcohol does to energy balance
Alcohol provides approximately 29 kJ per gram — more than carbohydrate or protein, less than fat. This caloric content is often underestimated, because alcohol is consumed in liquid form alongside social settings where food intake is also elevated.
A standard drink in Australia (10g of alcohol) provides roughly 290 kJ from the alcohol alone, before mixer or accompaniment. Four drinks in an evening adds approximately 1,200 kJ of alcohol energy before food, which is not negligible within a weekly energy balance.
Alcohol also appears to reduce the inhibition around food choices — there is reasonable evidence that alcohol consumption increases total food intake in the period surrounding drinking, independent of the direct caloric contribution.
The practical framing
None of this is an argument for eliminating alcohol. It is an honest account of what alcohol does to the metrics that Intentra tracks: sleep quality will be lower the night you drink; recovery markers (HRV, resting heart rate) will reflect this; energy intake tends to be higher on days you drink.
Knowing this, you can make informed choices and account for alcohol in context rather than treating it as invisible to your health data.