Fatigue is one of the most common complaints in general practice, and hydration is one of the first things clinicians ask about. The reason is simple: the effects of even mild fluid loss show up quickly in how we feel and think.

Studies on healthy adults have found that losing as little as 1–2% of body weight in water is enough to measurably affect mood, concentration and perceived effort during ordinary tasks. People report feeling more tired, more irritable, and less mentally sharp — often before they notice thirst.

How much water does someone actually need? The old "eight glasses a day" rule is a rough approximation, not a medical guideline. Most healthy adults do well on somewhere between 2 and 3 litres of total fluids per day, including what comes from food, coffee and tea.

A more practical target: drink enough that your urine is pale straw-coloured for most of the day, and increase intake during exercise, heat, illness or long flights. If you regularly feel sluggish in the afternoon, a glass of water is a cheaper first experiment than another coffee.