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I used to heavily underestimate the importance of drinking water to stay well hydrated, especially when I moved to Thailand. 

The high temperatures here, combined with humidity usually in the range of 70% or higher, made me dehydrate very quickly. Because the air is already saturated with water, sweat doesn't evaporate to successfully cool you down, and thus you perspire continuously.

Now it wasn't that I that I didn't drink any fluids, but rather that I wasn't drinking enough water. Because of the low cost of carbonated drinks, coffee, and flavoured teas I didn't see the need to drink water as I was already taking in fluids. How very wrong I was! After about 2 weeks or so I started observing some symptoms. I felt sleepy and had a general lack of energy, I noticed irregularities in breathing, I would get dizzy, and had urine of dark colour.

I took the time to look up the symptoms, and it struck me immediately! I couldn't believe how blind I was, when I should have known better. Needless to say, I stopped my bad habits and I now drink at least two litres of water per day. All of my previous symptoms were gone by the second day of staying properly hydrated with good old-fashioned water.

I hope this helps answer your question. Stay healthy and hydrated!! 

A sure way to gauge how hydrated, or dehydrated you are, is to monitor the darkness and clarity of your urine. 

If your urine is dark in color, and/or not very clear in opacity, it could be, and likely is a sign that you are somewhat dehydrated. 

It's recommended that you drink 2 liters of water, on average, a day, but everyone is different. I suggest listening to your body, and perhaps creating a hydration schedule for yourself to help stay hydrated. 

If you become lethargic, and faint, you may be extremely dehydrated, and should seek medical assistance if this is the case.

Also, be careful of over hydration !! Yes it is possible to drink too much, and is very dangerous!

For your convenience, here is an article about over hydration:

https://en.wikipedia.org/wiki/Hypernatremia

I hope this helps answer your question!

The signs and symptoms of dehydration in adults range from minor to severe.

Mild to moderate dehydration may include the following:

Increased thirst

Dry mouth

Tired or sleepy

Decreased urine output

Urine is low volume and more yellowish than normal

Headache

Dry skin

Dizziness

Few or no tears

The above symptoms may quickly worsen and indicate severe dehydration with signs and symptoms are developing; severe dehydration may include the following:

Severely decreased urine output or no urine output. The urine, if any, produced is concentrated and a deep yellow or amber color.

Dizziness or lightheadedness that does not allow the person to stand or walk normally.

Blood pressure drops when the person tries to stand after lying down (low blood pressure or orthostatic hypotension)

Rapid heart rate

Fever

Poor skin elasticity (skin slowly sinks back to its normal position when pinched)

Lethargy, confusion, or coma

Seizure

Shock