People lose weight during sleep through respiration (breathing) and perspiration (sweating). It’s estimated that during sleep, we lose 1.9 grams of weight per minute, which amounts to nearly 1 kilogram of weight per 8 hours of sleep. Most of the weight we lose during sleep is water weight that escapes our system.
Each time we exhale we lose a little bit of water. It’s easy to check – breath on a cool piece of glass and you will see some moisture.