When you look at yourself in the mirror first thing in the morning, you see a belly that's flatter than at any other time during the day. This happens because you're coming off of a multi-hour fast -- which means that most of your food has been digested, so it can't cause any bloat. You've also lost water weight overnight, which contributes to a thinner appearance and a lower weight. Many diets recommend that you weigh yourself in the morning because this is your "slimmest" time of day.
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Scale Weight in the Morning
Scales measure more than your fat mass -- scales reflect changes in weight caused by water retention, constipation, hormonal changes and your clothing. Your weight is usually lowest in the morning, because you haven't eaten or had much to drink. Your weight fluctuates by several pounds over the course of the day, depending on what you put into your body. It's impossible to gain that much fat in one day -- you'd have to consume 7,000 to 14,000 calories in excess to do so. When you're getting dressed, it's also easier to weigh yourself without clothes to further "reduce" your weight.
Water Loss Overnight Makes You Look Slimmer
You process water overnight, even if you're not sweating and urinating, so your body is slimmer. You lose water through your skin, from respiration as you breathe out moisture and through functions such as pumping blood and running your internal organs. Physician Margaret Polaneczky notes on her website that 80 percent of your morning weight loss is because you've lost water weight -- not including urine. Any urine you expel first thing in the morning is just another loss of water weight that leads to a slimmer feeling.
Some mornings your slimmer feeling will be more evident than on other mornings, mostly because of water retention. If you consumed a high-carbohydrate or high-sodium meal the night before, you might retain more fluid. A heavy weight-lifting session can also make you retain water the next day because of the swelling around the micro-tears in your muscles. If you're a woman, hormonal changes -- especially around the time of your period -- can also affect how slim you feel in the morning.
Food Digests During the Night
Overnight, you aren't putting food into your body, and the food you ate the day before has been mostly digested. The next morning you see a lighter weight and a slimmer shape. Anything you eat will affect your physical weight and the shape of the digestive tract during the day. As your body processes food, it retains some water, develops gas and produces stool. These can all contribute to feelings of a bigger stomach and heavier body. If you have an intolerance to lactose or another food, or if you suffer from Irritable Bowel Syndrome or you suddenly increase your fiber intake, you may feel unusually bloated during the day. This bloat subsides overnight as your digestive tract processes the food.
Height Increases Overnight
When you wake up in the morning, you're slightly taller, which may make you feel slimmer. Lying down makes the discs in your spine separate slightly, so your height will increase a small amount, as a result. Over the course of the day, the compression of walking squishes them back together to your "normal" height. This compression is why you lose the average of 1 centimeter in height when you run a marathon. If you're taller in the morning, even a little bit, your weight spreads out over a larger surface area and you feel slimmer.