What Causes Light Headedness In Morning Time?

Most mornings I feel a bit light headed and not quite dizzy but more sluggish or like I almost have a headache. This feeling seems to be worse if I exercise the night before. Generally it takes until after lunch to feel normal. I am 27yr old female, have no history of medical problems, am allergy free and eat regularly and keep well hydrated. I love sport and exercise. Any suggestions?