It’s 3pm, you feel flat and foggy, and you realise the only thing you’ve drunk since breakfast is half a cup of coffee. Can being a little low on water really make you that tired? The research says yes. It also says the amount of water you need is probably less than the “eight glasses” rule suggests, and that thirst is a better guide than most people give it credit for.
This article is part of our complete guide on how to increase your energy levels naturally. It covers what mild dehydration does to energy and mood, how much water adults actually need according to official guidelines, simple ways to check your own hydration, and when tiredness probably isn’t about water at all. It is general information, not medical advice.
Can Mild Dehydration Make You Tired?
Yes, and it doesn’t take much. The best evidence comes from two controlled experiments run by the same research group at the University of Connecticut, one in women and one in men.
The study in women: 1.36% dehydration, more fatigue
In a 2012 randomized, placebo-controlled study published in the Journal of Nutrition, Armstrong and colleagues tested 25 healthy young women (average age 23) on three separate days. On two of those days, they were mildly dehydrated through exercise, with or without a diuretic. On the third, they stayed fully hydrated. The researchers used a placebo pill so the women couldn’t tell which day was which.
The dehydration was small: an average loss of 1.36% of body mass. For a 60 kg woman, that’s roughly 800 mL of water, about what you might lose over a warm morning without drinking. Even so, compared with the hydrated day, the dehydrated women reported:
- lower vigour and higher fatigue on a standard mood questionnaire,
- worse overall mood,
- lower concentration, and tasks feeling harder than they were, and
- more headaches.
These effects showed up both at rest and during exercise. What didn’t change much was actual thinking ability. Most measures of cognitive performance were not affected, so the women could still do the work, it just felt harder and more tiring.
The study in men: 1.59% dehydration, more fatigue and anxiety
A companion study by Ganio and colleagues, published in the British Journal of Nutrition in 2011, used the same design with 26 healthy young men. Their average dehydration was 1.59% of body mass. The men reported more fatigue and more tension and anxiety. Unlike the women, they also made more errors on a visual vigilance task and were slower on a working-memory task.
The effects in both studies were modest. Nobody collapsed. But “a bit more tired, a bit grumpier, everything feels like more effort” is exactly the kind of low-grade fatigue that many people put up with for years without knowing why.
What these studies don’t show
It’s worth being clear about the limits. Both studies were short (single days in a lab), small, and done in healthy young adults. Neither shows that drinking extra water beyond normal hydration gives you more energy. The finding is that being slightly short of water makes you feel worse, not that more is always better. A large 2010 review of water and health in Nutrition Reviews (Popkin, D’Anci and Rosenberg) also noted that there were no long-term randomized trials in this area, and that’s still largely true.
How Much Water Do You Actually Need?
The two most widely used official guidelines are surprisingly close. Note that both refer to total water, from drinks and food, not just glasses of plain water.
| Source | Women (total water/day) | Men (total water/day) |
|---|---|---|
| European Food Safety Authority (EFSA), 2010 | 2.0 litres | 2.5 litres |
| US National Academies (Institute of Medicine), 2004 | 2.7 litres | 3.7 litres |
Both are “adequate intakes”, based on what healthy people typically consume, not minimum requirements. Food usually supplies about 20% of total water, so in practice EFSA’s figures work out to roughly 1.6 litres of drinks a day for women and 2.0 litres for men. The US report says drinks made up about 81% of total intake in its survey data. EFSA’s figures assume a moderate climate and moderate activity. You need more in hot weather, when you exercise and sweat a lot, during pregnancy (EFSA adds about 300 mL a day), and when breastfeeding (about 700 mL a day more).
What about “eight glasses a day”?
The famous 8×8 rule (eight 8-ounce glasses, about 1.9 litres) is not far off EFSA’s figure for drinks. But it isn’t based on research. In a 2002 review in the American Journal of Physiology, kidney physiologist Heinz Valtin of Dartmouth searched for scientific support for 8×8 and found none. He suggested the rule may have come from a 1945 US recommendation of about 1 mL of water per calorie of food, which went on to say that most of this is already in prepared foods. That part seems to have been forgotten. Valtin’s conclusion applied to healthy adults in a temperate climate who aren’t very active. He also pointed out that tea, coffee and other drinks count toward the total.
Does coffee count?
For most regular coffee and tea drinkers, yes. Valtin’s review and the US National Academies report both concluded that caffeinated drinks can count toward your daily total. The bigger energy question with coffee is when you drink it, not whether it dehydrates you (see our guide to caffeine timing). Alcohol is different: it shouldn’t be counted toward your fluid intake.
How to Tell If You’re Drinking Enough
You don’t need an app or a measuring bottle. For most healthy adults, three simple checks are enough.
- Thirst. In healthy younger and middle-aged adults, thirst is a reliable signal. Drinking when you’re thirsty, and with meals, covers most needs on an ordinary day.
- Urine colour. NHS urine colour charts treat clear to pale straw yellow as well hydrated. Dark yellow, amber or honey-coloured means you should drink more. Some vitamins (especially B vitamins), foods and medicines can change the colour, so don’t rely on this alone.
- Context. If you’ve been sweating, sitting in a hot room, travelling, ill with vomiting or diarrhoea, or drinking alcohol, you probably need more than usual, even before you feel thirsty.
Older adults need to be more deliberate
Thirst gets less reliable with age. In a brain-imaging study published in PNAS in 2008, Farrell, Egan and colleagues made healthy younger men (in their 20s) and older men (late 60s to early 70s) thirsty with a salt-water infusion. Both groups rated their thirst similarly, yet the older men drank only about half as much water as the younger men. Older adults also tend to have less total body water to begin with, so a small shortfall has a bigger effect. If you’re over 65, or caring for someone who is, regular drinks at set times (with meals, mid-morning, mid-afternoon) are more dependable than waiting to feel thirsty.
Can You Drink Too Much Water?
Yes. Forcing down large amounts of water you don’t need is not a fatigue cure, and in extreme cases it’s dangerous. Drinking much more than your kidneys can clear dilutes the sodium in your blood, a condition called hyponatraemia. Early symptoms include nausea, headache, confusion and, ironically, tiredness.
The clearest example comes from endurance sport. In a study of 488 runners at the 2002 Boston Marathon, published in the New England Journal of Medicine in 2005, Almond and colleagues found that 13% finished with hyponatraemia, and three had critically low sodium. The strongest risk factors were gaining weight during the race (which means drinking more than they sweated out) and taking more than four hours to finish. Whether runners drank water or sports drinks made no difference. The amount was what mattered.
For everyday life, the message is simple: drink to thirst, drink more when you sweat, and don’t treat litres of water as a goal in itself.
Easy Ways to Stay Hydrated Through the Day
- Start the morning with a drink. You’ve gone 7–8 hours without fluid. A glass of water, tea or coffee with breakfast gets you off to a good start.
- Tie drinks to routines. A glass with every meal, and one each time you refill your coffee or take a break, adds up without needing to count.
- Keep water where you work. People drink more when it’s within reach. A bottle on your desk works better than a good intention.
- Eat water-rich food. Fruit, vegetables, soups and yoghurt all contribute. This is the food part of “total water”.
- Replace what you sweat. After exercise or a hot day, drink until you’re no longer thirsty and your urine is pale again. For ordinary workouts, water is enough. Coconut water is a pleasant option if you like it, but it isn’t necessary.
- Go easy on alcohol. It increases fluid loss and disrupts sleep, which is a double hit to tomorrow’s energy.
When Tiredness Isn’t About Water
If you’re drinking normally, your urine is pale and you’re still tired all the time, dehydration probably isn’t the main cause. Common culprits worth looking at include low iron stores, which can cause fatigue even when you aren’t anaemic (see our article on low ferritin and fatigue), and poor-quality sleep, including undiagnosed sleep apnea. If you wake up tired after a full night in bed, our guide on why you feel tired even after 8 hours of sleep goes through other common causes.
The NHS advises seeing a GP if you’ve felt tired for several weeks without a clear reason, or if tiredness comes with other symptoms such as weight loss or low mood. Get medical help promptly if you have signs of significant dehydration that don’t improve with drinking, such as dizziness, confusion, very little urine, or a fast heartbeat, especially in an older adult or someone with vomiting or diarrhoea. Constant, unusual thirst is also worth mentioning to a doctor, because it can be a sign of diabetes.
The Bottom Line
Being even slightly dehydrated (around 1.5% of body weight) makes people feel more tired, less focused and more irritable, even when their thinking is mostly fine. Most adults need about 1.6–2 litres of drinks a day plus the water in food, more in heat or with exercise. Thirst and pale urine are reliable guides for most healthy people, but older adults should drink at regular times rather than waiting to feel thirsty. Drinking more than you need won’t give you extra energy. Water is one part of the fatigue picture, so look at sleep, iron, movement and stress too.
Frequently Asked Questions
Can dehydration make you tired?
Yes. Controlled studies found that losing as little as 1.36–1.59% of body mass through mild dehydration increased feelings of fatigue, lowered mood and made tasks feel harder in healthy young adults. Rehydrating is a simple first step if you feel flat and haven’t drunk much.
How much water should I drink a day to have more energy?
The European Food Safety Authority recommends about 2.0 litres of total water a day for women and 2.5 litres for men, from drinks and food combined. That’s roughly 1.6–2 litres of drinks. You need more in hot weather or when you exercise. Drinking more than you need won’t give you extra energy.
How quickly does drinking water help with tiredness?
If mild dehydration is the cause, drinking enough to bring your urine back to pale yellow is usually enough. There’s no fixed timeline, but the effects in studies were linked to short-term dehydration rather than lasting damage. If you’re well hydrated and still tired, water is unlikely to be the problem, and it’s worth looking at sleep, iron levels and other causes.
Is the 8 glasses of water a day rule true?
Not as a strict rule. A 2002 review found no scientific evidence behind “8×8”. The amount is close to official guidance for drinks, but tea, coffee, other drinks and water-rich food all count, and needs vary with body size, climate and activity.
What are the signs of dehydration?
Common signs include thirst, dark yellow urine, peeing less often, a dry mouth, headache, tiredness and feeling lightheaded. Seek medical help if you have confusion, very little urine, a fast heartbeat or dizziness that doesn’t improve with drinking.
This article is for general information and is not medical advice. Talk to a qualified healthcare professional if you have persistent or unexplained fatigue, if you’ve been advised to restrict fluids, or if you have kidney or heart conditions.
Sources
- Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. Journal of Nutrition, 2012;142(2):382–388.
- Ganio MS, Armstrong LE, Casa DJ, et al. Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition, 2011;106(10):1535–1543.
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific opinion on dietary reference values for water. EFSA Journal, 2010;8(3):1459.
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2004/2005.
- Valtin H. “Drink at least eight glasses of water a day.” Really? Is there scientific evidence for “8 × 8”? American Journal of Physiology – Regulatory, Integrative and Comparative Physiology, 2002;283(5):R993–R1004.
- Popkin BM, D’Anci KE, Rosenberg IH. Water, hydration, and health. Nutrition Reviews, 2010;68(8):439–458.
- Farrell MJ, Egan GF, et al. Effect of aging on regional cerebral blood flow responses associated with osmotic thirst and its satiation by water drinking: a PET study. PNAS, 2008;105(1):382–387.
- Almond CSD, Shin AY, Fortescue EB, et al. Hyponatremia among runners in the Boston Marathon. New England Journal of Medicine, 2005;352(15):1550–1556.
- NHS. Tiredness and fatigue (nhs.uk).
