How does regular exercise make us more alert and energetic

It increases our health status and when your healthy, your senses tend to be stronger. a person who runs up the stairs for the first time is probably going to be tired by the time he makes it to the top, as where if a person that runs up the stairs everyday will be perfectly fine because it is regular exercise. 

Her parents start noticing changes about her. she's probably just competive though not on drugs