Doctor can evaluate
An normal, healthy person who gets light-headed when getting up from lying or sitting, or who feels faint sometimes, especially on hot days, may have a problem of not enough blood returning to his head fast enough.
The blood pressure
may be normal or a bit low. After a doctor's exam to rule out medical problems, a person can try taking 3 ounces of extra water each hour, plus a bit of salty snacks.