Why do I lose weight when I sleep?

The main thing affecting overnight weight change is the amount of water in your system. Water weighs an amazing amount. A gallon of water, for example, weighs 8 pounds and a pint weighs 1 pound. In other words, if you drink a 1-liter bottle of soda, instantly you will gain 2.2 pounds, just from the water.

Overnight, two processes cause you to lose water gradually: breathing and sweating. In respiration, each time you exhale, you lose a small amount of water. If you exhale onto a cool piece of glass you will see this moisture. Through sweating, also called transpiration, you lose water through the skin. Over the course of one night, both of these processes eliminate more water than you might think.

Also contributing to the weight loss is the tradition of using the toilet prior to the morning weigh-in. That process alone can eliminate up to a pound of water.

The average person eliminates about 1.2 liters of water just in urine every day. Another liter a day is eliminated through perspiration and respiration. That's 2.2 kilograms (or almost 5 pounds) throughout the day.

When you weigh yourself prior to going to bed and then first thing in the morning when you wake up there is frequently some weight loss. This weight loss is primarily attributed to loss of water. We lose water many different ways. We sweat while we sleep, we lose water from the simple act of breathing and of course we lose water through our bladder. Since roughly 70% of our bodyweight is water then you can imagine the ocean of water that you are carrying around. Weigh yourself once per week at the same time. Pay attention to this weigh in over several weeks and you will be able to see a trend. This will be more indicative of what is going on with your body fat.