Why do we weigh a couple pounds less when we get out of bed in the morning compared to when we got in the night before?
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We breathe out a lot of moisture. And sweat, too.
And I bet you peed before you weighed yourself.
Think of your body like a car that is always running. It redlines when you sprint or do something really strenuous. It idles when you are sleeping. 24/7, it is burning fuel composed of sugars fats and carbohydrates to sustain the engine. Those chemicals combine with oxygen in your body making carbon dioxide and water plus energy as byproducts. At night, you continue to breath out that CO2 and H2O but you aren’t eating or drinking anything to replace it. So when you wake up you are lighter than when you went to bed because of the CO2 and H2O you exhaled overnight.
Your body is made up of water and when you sleep, you sweat and breathe out water. That’s why you weigh a little less when you wake up.
The common misconception is that any weight we lose is either water or food waste. Sure, those things are weight that can be lost in visible ways, but how does fat get lost? Our blood isn’t depositing things in our intestines or anything (as long as we are healthy), and the molecules of fat and carbs (carbon, hydrogen, and oxygen) are more than what makes up water (hydrogen and oxygen)!
Instead, our blood is depositing the waste of any fat or carbs we burn into our lungs: carbon, in the form of CO2 (ever wonder why we breath in O2 and breathe out CO2? where does the C come from? From our metabolism!).
And logically, the backbone of most organic material (carbon) weighs a surprising amount! Even if we usually think of air as weightless, there is enough mass difference between CO2 and O2 that it adds up over night.
Because we are breathing through the night (otherwise we’d be dead), we lose weight through the night. It is also part of why when we burn more calories (exercise) we breathe harder.
It’s probably easier to think of yourself as gaining weight throughout the day than it is to think of it as losing weight over night. In the morning it’s going to be closer to your true body weight since you haven’t eaten or drank anything in 8+ hours. As you eat and drink fluid throughout the day that will show up on the scale until you metabolize it and excrete out the waste.
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