Do you know it is because of the hot weather we often feel this? Why? Here's the reason..
Your body has to work more to maintain your average core body temp of 98.6 degrees. This means processing water to sweat and attempt to cool yourself. Since your body uses this energy, its less you get to use freely. Drink lots of water and supplement the water with electrolytes to maintain your energy. Don't forget to eat either. Lots of people lose their appetite in the heat and deprive their bodies of awesome vitamins and nutrients.
That is why, even we are fasting...during sahur we need to maintain our healthy lifestyle and diet. Wanna know a trick? Try wetting a towel and put it in the back of your neck...this works as an air conditioner to your body...and thus makes your body cool down a bit.
I almost got dehydrated when I was gone to Penang a few days ago, and it doesn't feel good. It hurts my right side of brain...thankfully I got home and rest for a while. I hope this info helps you, toodles!