We can’t live without water. It’s critical for proper body and brain function. It enables our blood to flow properly and helps us stay energized.

The good news: "Most people can handle mild amounts of 'dehydration,’” says Dana S. Simpler, MD, an internist in private practice in Baltimore. “The body regulates fluid balance through a number of complex pathways, including reduced urine production, increased reabsorption of water in the intestines and thirst — which prompts rehydration."

Simpler is careful to point out the difference between being low on fluids and clinical dehydration. "Dehydration mainly occurs with illnesses such as fever, diarrhea and vomiting where excessive fluids are lost and not being replaced or in a sport situations/hot weather where excessive perspiration is not being replaced," she explains.

So how much water do you need? And what about that old standby recommendation to drink eight 8-ounce glasses of water per day? “It’s an oversimplification of what the body actually needs,” says Simpler. "There is really no 'optimal' amount of hydration.” The weather, your diet and exercise habits are all a factor in how much water you need. Some days you’ll need more fluids, other days less.

Those fluids can come from virtually any drink and even some foods. Milk, smoothies, and juice drinks count. Caffeinated beverages, such as tea, coffee and energy drinks, count, too. Eating water-containing foods, including tomatoes, cucumbers, watermelon and oranges, is also a great way to up your fluid intake.

All things considered, it makes sense to clue in to symptoms that can signal the need for more fluids. Here are seven signs to pay attention to.

#1: You’re thirsty.

It may seem obvious, but a dry mouth and how thirsty you are do correlate to how hydrated you are.

#2: You’re hungover.

Processing alcohol requires water. If you overdid it last night, you need to drink as much as a quart or more water than you usually drink, says Ralph E. Holsworth, DO, an osteopath at Southeast Colorado Hospital in Springfield, CO. Even better, head off dehydration in the first place. “If you’re going to drink alcoholic beverages, try to drink one eight-ounce glass of water for every alcoholic beverage you consume to help reduce your dehydration risk,” he advises.

#3: You’re lightheaded or worn out.

If you feel dizzy, you may be dehydrated. First, stop your activity and rest, says Lisa Katic, RD, a registered dietitian in Washington, DC. Reach for fluids such as water or a sports drink with electrolytes, which replaces both lost fluids and minerals. Drinking should help you feel steadier.

#4: Your urine is extra dark.

Concentrated urine is a clear indicator that you’re dehydrated. It means your urine has more waste in it. (Staying hydrated makes it easier for waste to be flushed out efficiently.) “Ideally, your pee should be pale yellow,” says Holsworth. “If it’s dark yellow, you’re probably not drinking enough.” Katic says the recommendation is to drink six ounces of water within an hour if you notice your urine is darker than normal.

#5: You’re tired.

If you’re just plain pooped, dehydration may be to blame. Our blood and bodily fluids are made up mostly of water, so if you’re extremely dehydrated, your heart has to pump harder to carry blood throughout your body. The result: intense fatigue, says Katic.

#6: You can’t concentrate.

Dehydration can often impair your ability to focus on a certain task. “By hydrating, you’ll do wonders to improve your mental acuity,” Katic says. In fact, even mild dehydration can alter your mood and energy levels, according to two recent studies conducted at the University of Connecticut’s Performance Laboratory.

#7: You’re constipated.

The better hydrated you are, the better you’ll digest your foods, Katic says, explaining that water helps waste move along from your stomach through your intestines. If you’re dehydrated, your stools may be harder, making them more difficult to pass. To prevent constipation, don’t forget to eat plenty of fiber in addition to getting enough fluids.

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Lambeth Hochwald is a New York City-based journalist who covers such topics as family, health and parenting. She is a current contributor to Dr. Oz: The Good Life, Everyday With Rachael Ray and Yahoo Parenting.