Your body simply cannot function properly if it is dehydrated – and the more dehydrated you are, the more problems will occur. All throughout the day, and even while we sleep, our body uses water. The “water” we see expelled through our sweat and urine is essential to removal of toxins, and we actually need more than the amount we put out to make up for the losses. This is because every single cell in our body uses H2O to function. That doesn’t even count the activities we participate in everyday, like exercise and yes, even thinking. Our brain cells need a lot of water in order to function.
1. Dark, Infrequent Urine:
This one is probably the most obvious and best signs. Ideally, you should always aim to be observant regarding the color of your urine as it is an immediate indicator of your hydration level. Your urine should be pale in color. If it is dark, it is too concentrated and you need more fluids. Also if you haven’t urinated for several hours, you probably are not drinking enough.
2. Bad Breath:
Without enough water, our bodies start to withhold whatever water it can, wherever it can. Feeling extremely hot yet being unable to sweat, or being unable to pee, are signs of severe dehydration, but these signs occur later on. One of the first parts of the body that get depleted of water is the mouth. Xerostomia, or dry mouth, can cause a foul odor to build up in our mouths due to the lack of saliva and its cleansing properties. 
Drinking water is also a good way to wash off the plaque and bacteria that build up in your mouth. So the next time you’re offered a mint, you might want a glass of water or two to go with that.
3. Dry Skin:
Dry skin, especially on the lips and eyes, are clinical signs of dehydration. When your skin is thirsty, chances are most of the cells inside of your body are getting thirsty as well. Moisturizers and creams don’t work as well as getting enough H2O every day to fix dry, dull skin.
Regularly drinking water also fixes premature wrinkles and fine lines. Taking a water bottle from home with you wherever you go might help you get your water fix daily.
4. Muscle Cramps:
Muscle cramps are a sign of a great many alarming things, including dehydration. The most common cause of cramps is insufficient nutrients like magnesium, potassium, or sodium, but you can have a lot of these in the diet without enough water to transport them to the muscles that need them.
5. Headaches and/or Irritability:
Your body may be in need of more water if you’re suffering an unusual headache. If you’re finding tasks more difficult than usual, or if you’re feeling slightly moody, then it might be time for a break and a glass of water as well.  Difficulty concentrating is also a sign that you’re slightly low on H2O.
We all know that the number one culprit for constipation is usually lack of fiber. In some cases, however, lack of water is more of a problem than lack of green leaves in the diet.  The next time you’re having trouble with bowel movement, drink a couple of glasses of water and see if it helps.
How Much Water Do I Really Need?
Recommended daily intake of water may vary per region, and this may be because climate also affects how much water you need a day. In the U.S., the typical daily water intake for males is 3.7 liters and 2.7 liters for females. In the UK, the recommendation is 1.8 liters for both.  In the desert, you’ll need more. Keep an eye on your urine color.
8 glasses a day with a 240 mL glass measurement is a more popular rule of thumb, and it helps a lot of people remember to stay hydrated throughout the day. People with renal problems should consult with their doctor for their recommended daily water intake.
 Bad Breath (Halitosis.) Patient.co.uk. http://www.patient.co.uk/health/bad-breath-halitosis
 Cramps. Wikipedia. http://en.wikipedia.org/wiki/Cramp
 Mild Dehydration Affects Mood in Healthy Young Women. The Journal of Nutrition. http://jn.nutrition.org/content/142/2/382.short
 Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition. http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8425835&fileId=S0007114511002005
 Mild dehydration: a risk factor of constipation? European Journal of Clinical Nutrition. http://www.nature.com/ejcn/journal/v57/n2s/full/1601907a.html
 Drinking Water. Wikipedia. http://en.wikipedia.org/wiki/Drinking_water