Your brain on a 1% water deficit

After seven or eight hours of sleep, you've gone the longest stretch of your day without water. Then most of us reach for coffee. It's a small habit, but research suggests it may be costing you more focus than you think.

A small deficit, a noticeable effect

In a 2012 study published in the Journal of Nutrition, researchers induced mild dehydration in healthy young women, a fluid loss of just over 1% of body weight, the kind you can reach through a normal morning, a workout or a warm day.

Compared with when they were well hydrated, the women reported lower mood, more fatigue, more difficulty concentrating and more headaches. They also perceived tasks as harder. Their performance on most cognitive tests held up, but how they felt doing them changed noticeably.

One percent is roughly the point where many people are only starting to feel thirsty. By the time you notice, it's already affecting how you feel.

Why water alone isn't the whole story

Hydration isn't just about volume. Your body needs electrolytes, minerals that carry an electrical charge, to keep fluid where it belongs and to power nerve and muscle function.

• Sodium helps your body hold on to fluid and maintains blood volume. It's the main electrolyte you lose in sweat.
• Potassium works alongside sodium to regulate fluid balance inside your cells and supports muscle contraction, including your heart.
• Magnesium is involved in hundreds of processes, including energy production and muscle relaxation.

Drinking large volumes of plain water quickly can mean much of it passes straight through. Small amounts of sodium and potassium help your body absorb and retain what you drink.

Women and hydration

Hormones affect fluid balance. Across the menstrual cycle, shifts in estrogen and progesterone change how the body regulates water and sodium, and some women notice they feel more dehydrated or bloated at different points in the month. During perimenopause, hot flashes and night sweats can increase fluid loss too.

A better morning sequence

• Water first. A full glass before your coffee replaces some of what you lost overnight.
• Add electrolytes when it matters. After a sweaty workout, on hot days, or when you're running low on sleep, a pinch of salt or an electrolyte drink can help.
• Eat your potassium. Fruits, vegetables, beans and yogurt are excellent sources.
• Check the colour. Pale straw-coloured urine is a simple, reliable sign you're on track.

Focus starts with the first glass of the day.

A note on balance

More isn't always better. Very high fluid intake without electrolytes can be harmful, and people with heart, kidney or blood-pressure conditions may need to limit sodium. If that's you, follow your doctor's advice.