without going to the bathroom, a person naturally loses over while they sleep at night. one reason is humans breathe in oxygen but breathes out carbon dioxide. that is breathing in one molecule of oxygen for every two molecules of carbon each time they breathe out. a molecule of carbon does not weigh much by itself, but given that with each breath twenty billion-trillion molecules are expelled, over a 7-8 hour period that adds up to over a pound of carbon. normal perspiration occurs while sleeping but usually only causes a small amount of water weight loss.