Being dehydrated can make you tired, cranky and sick

This article was previously published on March 29, 2018 and has been updated with new information.

Have you ever been so busy that you dropped even one or two sips of water for an extended period of time, and then suddenly realized that you were incredibly thirsty and needed a large sip of water? Replenishing your body’s water supply when it tells you you are thirsty can often help you avoid dehydration. In fact, your body’s physiological thirst mechanism is usually activated before you’re dehydrated, giving you a chance to rehydrate before it’s too late.

There are exceptions to this rule, however, with the elderly and young children in particular at risk of becoming dehydrated. It is estimated that 20% to 30% of older adults are dehydrated,1 often due to lack of water and the fact that people naturally have less water in their bodies as they age.2 Babies and children can also become dehydrated quickly, especially if they are sick and have vomiting or diarrhea.

One study even suggested that more than half of American children are dehydrated, while about a quarter don’t drink water every day.3 Among healthy adults, the National Academy of Sciences concluded, “The vast majority of healthy people adequately meet their daily hydration needs by letting thirst be their guide.”4

However, if you ignore your thirst or find yourself not drinking enough water during hot weather, especially when you’re exercising, it’s quite easy to become mildly to moderately dehydrated, with signs and symptoms that may surprise you.

Why your body needs water

Your body is made up of approximately 42 liters (44.4 liters) of water, which makes up between 50% and 70% of your body weight. Your blood is 85% water, your muscles 80% water, your brain 75% water and even your bones are 25% water.5 indicating how important this fluid is to your health. So what happens if you don’t drink enough?

The No. 1 risk factor for kidney stones is not drinking enough water to begin with. There is also some research showing that high fluid intake is associated with a lower risk of certain types of cancer, such as bladder and colon cancer.6 Even the risk of fatal coronary artery disease has been linked to water intake, with a study showing that women who drank five or more glasses of water a day reduced their risk by 41% compared to women who drank less. Men, meanwhile, reduced their risk by 54%.7

Your body also needs water for circulation, metabolism, regulation of body temperature and removal of waste. If you’re dehydrated, even mildly, your mood and cognitive function can also suffer. In a study of 25 women, those suffering from 1.36% dehydration experienced worsened mood, irritability, headaches and lower concentration, and perceived tasks as more difficult.8

Not drinking enough water can also put you at risk on the road, according to a study published in the journal Physiology & Behavior that found that dehydrated drivers made twice as many mistakes during a two-hour drive compared to hydrated drivers.9

How your body reacts to too little water

Your body works optimally when it is adequately hydrated, while negative biological changes occur when fluid is lacking. When you are dehydrated, the cerebrospinal fluid decreases,10 lead to changes in brain volume. Dehydration also affects your heart. In a March 2022 study, researchers found that maintaining proper hydration throughout your life “can slow the decline of heart function” and even reduce the incidence of heart failure.11

Your blood also thickens and circulates less, which can lead to muscle cramps and also causes your kidneys to retain water, which decreases your urine output. Further, according to Toby Mündel, senior lecturer in sports and exercise sciences, Massey University, New Zealand:12

“The thicker and more concentrated your blood becomes, the harder it is for your cardiovascular system to compensate by increasing the heart rate to maintain blood pressure. When your dehydrated body is ‘pushed’ – such as when exercising or confronted with heat stress – the risk of exhaustion or fainting increases. This can cause you to faint, for example if you stand up too quickly.

Less water also hinders the body’s attempts to regulate temperature, which can cause hyperthermia (a body temperature well above normal). At the cellular level, ‘shrinkage’ occurs as water is effectively borrowed to maintain other supplies, such as the blood. The brain senses this and causes an increased sensation of thirst.”

Mündel recommends tracking your body weight to monitor your hydration level. When you get out of bed in the morning, weigh yourself three mornings in a row and then average your weights. This is your normal baseline weight and you should stay within 1% of that if you are adequately hydrated (assuming other factors have not affected your weight).

Surprising Signs of Dehydration

When your body is dehydrated, the lack of water can manifest itself in surprising signs and symptoms, including:13

Bad breath – Saliva is antibacterial, but when you’re dehydrated, you have less saliva in your mouth. This allows odor-causing bacteria to thrive.

Craving for sugar – Thirst can disguise itself as hunger, and many people reach for a snack when they are actually thirsty. Sugar cravings are especially common when you’re dehydrated, because your liver, which releases stored glucose, needs water to do this.

Further, Amy Goodson, a sports dietitian for the Dallas Cowboys, told Health, “When you exercise in a dehydrated state, you use glycogen (stored carbohydrates) at a faster rate, depleting your stores more quickly.”14

Athletic decline — When you’re in the middle of a workout, a 2% decrease in body weight from water loss can lead to a 10% loss in performance, Goodson said.

Decreased alertness and increased fatigue — In a 2013 study, 20 healthy women in their mid-20s went off alcohol for 24 hours. Although no clinical abnormalities were observed in the biological parameters (urine, blood and saliva), thirst and heart rate increased and urine output was drastically reduced (and darker).

As for mood effects, the authors noted, “The significant effects of [fluid deprivation] on mood included decreased alertness and increased sleepiness, fatigue and confusion.”15 Other research has shown that even dehydration levels as low as 1% can adversely affect cognitive performance.16

Chills — If you feel cold for no reason, it could be because you need to drink some water. When you’re dehydrated, your body restricts blood flow to your skin, which can make you feel cold.

Constipation is another consequence of not drinking enough water, as your body draws water from your stool to make up for what you don’t take in. This, in turn, makes your stool drier, harder, and harder to pass. Other symptoms of mild and severe dehydration include:17

Mild to moderate dehydration Severe Dehydration
Dry, sticky mouth Extreme thirst
Drowsiness or fatigue Irritability and Confusion
Dry skin deep-set eyes
Headache Dry skin that doesn’t bounce back when you squeeze it
Dizziness Low bloodpressure
Dizziness high heart rate
Little or no tears when crying fast breathing
Minimal urine No tears when crying
Dry, cool skin A fever
muscle cramps Little or no urination and any urine color that is darker than normal
In severe cases, delirium or unconsciousness

How much water is enough?

There is quite a bit of debate about how much water the average person needs to stay healthy. You’ve probably heard the recommendation to drink eight 8-ounce glasses (or 8×8) of water a day to stay healthy, and it’s often stated as scientific fact. It’s not that simple, though, as many factors affect how much water you need, from your age and health status to your activity level and climate.

Furthermore, Dr. Heinz Valtin of Dartmouth Medical School in a review published in the American Journal of Physiology found no scientific basis for the 8×8 rule, which is more aptly described as a myth.18 Valtin also debunks some myths about water consumption, such as that waiting to drink until you’re thirsty is too late because you’re already dehydrated.

As Valtin said: “[T]hirst is so sensitive, fast and accurate that it is hard to imagine that evolutionary development has left us with a chronic water shortage that must be compensated by forced fluid intake.”19 Ultimately, you don’t have to get stuck figuring out the exact amount of water your body needs or keeping track of how many glasses you drink in a day.

You don’t need to, because your body will let you know. Simply using thirst as a guideline for how much water to drink is an easy way to make sure your individual needs are met, day by day. You can also use the color of your urine as a guideline. If it’s a deep, dark yellow, you’re probably not drinking enough water. A pale straw yellow color or pale yellow is typically indicative of adequate hydration.

If you have little urine or if you have not urinated for many hours, that is also an indication that you are not drinking enough. (Based on the results of a few different studies, a healthy person urinates an average of seven or eight times a day.)

If you know that you tend to ignore your thirst and not drink when your body gives you this sign, then it would make sense to pay more active attention and always take the time to drink some water when you are thirsty. In infants and children, detecting dehydration can be trickier, but if you see these symptoms, you can assume that your child is dehydrated and should seek immediate medical attention:

Sunken soft spot (fontanelle) on their heads

Little or no tears when they cry

Dry mouth

Few wet diapers


rapid breathing