If you’ve ever weighed yourself before bed at night and again in the morning, you may have been surprised that you lost anywhere from 1-3 pounds. This is fairly normal. We all lose weight at night while sleeping.
How much we lose overnight depends on whether we get enough sleep, how much water we excrete, and our basal metabolic rate (BMR), which is largely determined by the amount of muscle we have (muscle is quite metabolically active).
The body is in an entirely aerobic state while we’re sleeping, since it uses only fat to operate in such a low-energy state (carbohydrates are used only for higher energy demands). Our heart, brain and other organs all need energy to maintain basic functions while we’re sleeping.
In one hour of sleep, most people burn 0.4 to 0.5 calorie for every pound of body weight. For example, a 150-pound person would multiply 150 by 0.4 or 0.5 to get 60 to 75 calories used in one hour of sleep. After eight hours of sleep, that person has burned 480 to 500 calories.
A 200-pound person would burn 80-100 calories per hour, or 640-800 calories during eight hours of sleep.
This may seem unbelievable, but it’s true. You’re burning calories all day long — even while sleeping — just to stay alive; that’s your BMR. Additionally, energy is also expended by digesting your food and from non-exercise daily movements, such as brushing your teeth, getting dressed and pouring your morning coffee. This additional expenditure is known as your resting metabolic rate (RMR).
For American women, the average BMR is 1,493 calories. For American men, the average BMR is 1,662 calories.
Your BMR accounts for about 70 percent of the calories you burn every day (in general, men have a higher BMR than women). This is critically important since there is no way we can exercise away all the calories we consume each day.
Muscle is the single most important predictor of how well you metabolize your food, burn calories, and burn body fat. However, beginning in our late 20s, we start losing muscle, which means our metabolism also starts declining. A consistent weight training program can stop this decline and even reverse it.
There are approximately 3,500 calories in a pound of stored body fat. If you subtract 3,500 calories each week through diet, exercise or a combination of both, you will lose one pound of body weight.
On average, 75 percent of this is fat and 25 percent is lean tissue, which is why it’s so important to strength train while attempting to lose weight; you don’t want to lose your precious muscle along with that unwanted fat. Muscles at rest burn three times more calories than fat, so maintaining or increasing muscle mass influences BMR.
In simple terms, if you’re seeking to lose weight, aim for a 500-calorie deficit each day of the week (7 x 500 = 3,500).
As a guide, the American College of Sports Medicine (ACSM) recommends that women never drop their calorie-intake level below 1,200 per day and men should not go below 1,800 calories per day.
So, back to where we started: it’s possible for a 150-pound person to lose about 1 pound each week simply by sleeping eight hours every night. Remember, a person of this weight burns roughly 500 calories while sleeping each night. Sleep is very valuable!  In fact, sleep deprivation has been linked to an increase in weight.
Clearly, much of this 1-pound of weight loss is water, not just fat (we lose water gradually each night through respiration and perspiration), but burning 500 calories overnight while sleeping is no small matter.
The average person eliminates about 1.2 liters of water in urine each day, and also eliminates about 1 liter a day through perspiration and respiration. That’s 2.2 kilograms (almost 5 pounds) of weight fluctuation happening throughout the day.
So, don’t get caught up in the daily fluctuations of your body weight. You can gain up to 5 pounds during the day just from your food and beverage intake — but it’s not all fat!
Your BMR, non-exercise daily movements, and your daily exercise routine should allow you to at least maintain your present weight. To lose weight, however, you’ll have to reduce your daily caloric intake.
You weigh less in the morning than during the rest of the day because you don’t (normally, at least) eat and drink during the night. Remember that two cups of water (16 ounces or one pint) weighs one pound, so drinking a tall glass of water will temporarily be reflected in your weight.
The most accurate time to weigh yourself is first thing in the morning, when you have an empty stomach. Weighing yourself while naked will also give you a more accurate reflection of your weight. 
Lastly, always weigh yourself on the same scale. Even if it’s off by a pound or so, it will be relative to itself and still reveal any upward or downward movements. Different scales may give you different results, which can be misleading or even maddening.