What causes feeling of thirst?

You feel thirsty when your body loses more water than the amount you take in. Thirst receptors at the back of your throat will send signals to the brain when it's time for you to drink up. So whether you sweat alot in humid weather or doing strenuous physical activities like exercise, don't forget to get your much needed refill of fluids.

Water intake advice:
2 glasses upon waking up - activates internal organs
1 glass before meal - helps in digestion
1 glass before bedtime - avoids heart attack and stroke