When you play sport your muscles produce heat which causes you to feel hot and start to sweat. Therefore it is important to make sure you are well hydrated before playing sport and to drink during and after sport to replace fluids lost via sweat.
If exercising or playing sport for more than 30 minutes then it’s important to drink some fluid. Water is sufficient for sessions up to an hour, but for longer sessions isotonic sports drinks may be useful.
The evaporation of sweat from the skin surface allows the body to cool down and maintain its temperature within the narrow limits required. Sweat rate is reduced if you are dehydrated so the body is less able to control its temperature and this will impact on the ability to perform well in sport.