Your heart rate sometimes varies from its norm, temporarily, for many reasons. In hot weather, dehydration tops the suspect list.  With less fluid in the body, the heart must pump harder to get blood to your muscles. Even in moderate temperatures, you might still suffer from dehydration if you consume a little too much alcohol the night before. Assuming you do not take too many vitamins, an easy way to tell if you are dehydrated involves checking the color of your urine.  Stark yellow typically indicates dehydration.  As a guideline, your body absorbs one liter of fluid per hour.  Electrolyte drinks are preferable to straight water to prevent a condition called hyponutremia (Hyponutremia is a condition where the body gets too much water and not enough electrolytes. It can be as dangerous or more dangerous than dehydration). Other reasons for an elevated heart rate include lack of sleep, mental stress, or onset of a virus or other illness.