When we’re looking at how much weight people lose overnight there are two possible ways to answer. The first question is one of how many calories that you burn overnight and the other is how much less physical weight you lose, which is of course typically water weight.
There are a few variables to consider when we answer both of these questions, the fact of the matter is that both of these questions depend on the size of the person, but also the in the case of water weight, the heat of the room.
And whilst we may think overnight weight loss is simply down to sweat, there is actually a lot more to it. And of course sleep itself effects hunger through the day as well as general energy levels and has a lot more connections to weight loss and even preserving muscle.
How much water weight do you lose overnight?
On average we’re looking at anything between 200-500ml which is about half a pound to a pound of weight. Of course, this depends on a heat, and what you’ve consumed before bed. Alcohol or spicy food for example can make you sweat out more in your sleep. 
Another significant factor, and quite a surprising one to most people, when it comes to overnight weight loss is water vapor. As you sleep, your body naturally loses water through the process of evaporation. When you exhale, the warm air leaving your body carries away some of its moisture, resulting in a loss of water weight. On average, you can lose around 1-2% of your total weight during a night's sleep due to water vapor alone. Which may not sound like a huge amount at first, but when you consider a 180lb man, that can actually be 2lbs, on top of the overnight water weight loss from sweat. 
Lastly, a regular trip to the bathroom can also contribute to overnight weight loss. As we sleep, our bodies continue to produce urine, which needs to be eliminated. The older we get, the more urine we produce, leading to increased water loss during sleeping hours.
How many calories do you burn in your sleep?
To understand how many calories you burn whilst sleeping we need to understand BMR. BMR or basic metabolic rate refers to the number of calories your body needs to perform essential functions at rest. In other words, it's the minimum amount of energy required to keep your body alive. 
Then we have resting metabolic rate  which is similar to BMR but takes into account additional activities throughout the day, including sedentary behaviors. RMR accounts for approximately 60-75% of your daily energy expenditure. During sleep, your body's RMR remains relatively stable, contributing to the overall calories burned during this period.
On average though if we assume that your resting metabolic rate is 1800 calories, then you’d expect to burn 600 calories whilst you slept.
Factors Affecting Calores Burned During Sleep
While the average number of calories burned during sleep is relatively consistent, several factors can influence individual variations in caloric expenditure. It's important to note that these factors are not solely related to sleep but can impact your overall metabolism and energy balance.
Body Composition - Muscle mass plays a significant role in calorie burning. Individuals with higher muscle mass tend to have a higher metabolic rate, both during sleep and through the day. This is because muscle tissue requires more energy to maintain compared to fat tissue. 
Age - Age is another factor that can affect caloric expenditure during sleep. As we age, our metabolic rate tends to decrease due to a decline in muscle mass and hormonal changes. This decrease in metabolic rate can result in a lower number of calories burned during sleep. 
Physical Activity - Physical activity levels throughout the day can impact the number of calories burned during sleep. Regular exercise can increase your metabolic rate and promote calorie burning even at rest. Individuals who engage in regular physical activity may experience a slight increase in calorie burn during sleep. 
Sleep Disorders - Sleep disorders such as sleep apnea and insomnia can impact caloric expenditure during sleep. Sleep apnea, a condition characterized by pauses in breathing during sleep, can lead to disruptions in sleep quality and decrease overall calorie burning. Similarly, insomnia, a sleep disorder characterized by difficulty falling or staying asleep, may affect the quality and duration of sleep, potentially influencing caloric expenditure. 
So how much weight do you lose whilst sleeping?
All in all you should expect to wake up between 1-4lbs lighter, but this isn't your "weight loss" so to speak, the amount of calories you lose will be closer to 500, meaning that's about 1/7th of a pound of fat. This isn't quite as exciting as stepping on the scales and being substantially lighter, which is why it's important to maintain a consistent schedule when weighing yourself.
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