The science
In one study, men who lost about 1 to 2% of their body weight in fluid, a level you could hit without ever feeling properly “thirsty,” showed measurably higher fatigue and tension, plus worse working memory, than when they stayed hydrated (PubMed, Ganio et al., British Journal of Nutrition).
So what
1 to 2% doesn’t sound like much. For someone around 70kg, that’s under a kilogram and a half of fluid, easy to lose over a normal day: talking a lot, sitting in a warm office, skipping water because you were busy. No dramatic sweating required. That fog that hits around 3pm and gets blamed on lunch or bad sleep? Sometimes it’s just this.
Bottom line
Dehydration isn’t the only cause of fatigue, and electrolytes aren’t a cure for being tired. But it’s a cheap thing to rule out. If you’re flagging by mid-afternoon and can’t remember your last proper drink, that’s worth fixing before you reach for more coffee.
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