Whenever I weighed myself the night before, I always wondered how & why I’d lost weight by the time I climbed on the scale the next morning.
I finally decided to satisfy my curiosity & track down those answers, and I share them in this article.
Here’s why you weigh less in the morning
Depending on your weight, your body burns several calories as part of its nightly routine while you sleep.
You also lose water weight overnight from your breathing & sweating.
And all this occurs over a several hour period where you’re not eating or drinking anything either.
What’s next
Up ahead, we’ll look at how much our weight can fluctuate during a single day’s time, and why that occurs.
I’ll also share what medical experts have to say about weighing yourself – when’s the best time & why it’s important.
Science resources included
As is my custom here on heydayDo, I will provide links to all of the relevant sports science & medical resources, clinical studies, and nutritional data used in this article.
Overnight weight loss explained
Let’s look at each of these three main contributors to losing weight while we sleep in a little more detail:
- calories burned overnight
- water loss while sleeping
- overnight’s length of time between meals
Overnight calorie burning
Our bodies keep on working even while we’re asleep, doing countless tasks & processes that go beyond circulating our blood and making us breathe.
It just does it at a slightly lower rate of activity compared to when we’re awake.
Multiple sleep studies conducted over the past few decades, including these two here & here, have all demonstrated that our metabolism slows down while we’re asleep.
Yet it still operates at anywhere from 80-85% of its normal rate for healthy people who are getting sufficient hours of quality sleep.
Bottom line: All of these bodily functions need energy to be conducted, and the result of all of this system activity is that calories are burned while we’re sleeping.
(Later in this article I get into the specific numbers of how many calories each of us burn while sleeping.)
Overnight water loss
While we sleep (and while we’re awake too) our bodies excrete water as part of their normal system maintenance duties.
Two of the three main ways water leaves our bodies happen all night long while we sleep, and they’re known scientifically as respiration & transpiration.
We usually call them breathing & sweating.
From breathing, sweating, & peeing
Water leaves with every exhale, along with carbon molecules too (our bodies inhale oxygen and exhale oxygen + carbon dioxide).
And even though we don’t notice it since we’re not dripping with sweat, our bodies also excrete water through our skin throughout the night.
The third way we our bodies get rid of unwanted water is urination.
So if you ever pee between an evening weigh-in and a morning weigh-in on your scale, you’ll be a pound lighter for every 16 ounces you pee out.
Bottom line: Water is leaving your body all night long, through all the breathing, sweating, & peeing it does from the time of your last sip of liquid the night before to your first sip the next morning.
Several hours with no eating or drinking
Consider how many hours go by between your dinner the night before and the next time you get something to eat the following morning.
For most people that’s a period of anywhere from 10-12 hours.
And keep in mind that whether you’re sleeping or awake during that “no calories in” time frame, your body is continuing all of its calorie-burning and water-excreting processes.
Combining the calories burned with the water loss process I discussed above can easily translate to a loss of weight overnight, especially since you’re not eating or drinking anything during that time.
How much our weight fluctuates in a day
A 5 to 6 lb. fluctuation in weight for a regular adult within a single 24-hour period is not uncommon, says the Cleveland Clinic.
And a 2-4 lb. difference in your weight can be observed sometimes with just a few hours in between you weighing yourself twice on the same day.
Weight fluctuations are also common during the week for people whose weight otherwise stays the same over a period of months.
Researchers in this study recorded the daily weights in adults over a period of several weeks and found recurring patterns.
It’s easy to gain weight on the weekends
The people studied tended to weigh the most on Sunday into Monday, and had lowering weight beginning Tuesday that continued through the work week.
On Friday mornings they weighed their lowest, before the weight began increasing on Saturdays, thanks to more calories consumed Friday night.
I think anyone can take a quick glance at our society and see why we as a culture would likely weigh more at the end of our weekends compared to later in the work week.
We tend to go out more on the weekends, which leads to eating out more and drinking more alcohol & sugary beverages.
With more free time on the weekends for most nine-to-fivers, there’s more available snack time too.
Why your weight fluctuates so much
The main reason why your weight fluctuates so much during any given day is that there are a number of constantly changing factors that influence the number you see on your scale.
Throughout the day while you’re eating & drinking & doing this or that, your body is also constantly processing all of the above as well on its own schedule.
Consider the varying amounts of:
- food you’ve eaten
- water you’ve drank
- other beverages you’ve consumed
- exercise you’ve had
- sleep you had overnight
- sodium you’ve consumed
- sugary things you’ve had
- processed foods & drinks you’ve consumed
- water you’ve either peed or sweated out of you
All of these affect your body’s processing systems differently, and it’s constantly storing or excreting everything you put into it.
Drink a glass of water, gain a pound…
(…temporarily)
A good example of this is a 16 oz. glass of water.
If you drink that within a short period of time, you’ll see your scale weight go up by a pound.
Now depending on how efficiently your kidneys are processing your fluids at that time, you may either hold onto that water weight for awhile or pee it out within as little as a half hour’s time.
The best time to weigh yourself
A number of physicians with expertise in weight loss & weight management all agreed that the best time to weigh yourself is in the morning.
“In the morning”, as in getting on the scale right after waking up and before eating or drinking anything.
From Dr. Holly Wyatt, Medical Director at the Anschutz Health & Wellness Center – University of Colorado:
“Make it part of your morning ritual…You need to know that number on a consistent basis to help you manage your weight…” (3)
From Liz Salada MD, a board-certified internal medicine physician practicing in San Diego, CA:
“The best time to weigh yourself is usually in the morning on an empty stomach as your body is in a stable state, since typically you haven’t had anything to eat or drink for several hours.” (4)
Overnight weight loss FAQ
Here are answers to a couple more questions related to the natural process of losing weight while we sleep.
Does my body burn fat overnight?
In this research study funded by the National Institute of Health (NIH), doctors and scientists determined that good deep sleep is one of the best fat burners we have at our disposal.
For maximum benefit of this natural phenomena, the study found that the ideal number of sleep hours was 8 ½.
The downside of this study was also true: the participants who only got 5.5 hours of sleep a night had their natural fat-burning process reduced by over 50% compared to the 8.5 hour group.
How many calories do you burn sleeping?
Overnight calorie burning varies, according to a few factors:
- how much you weigh
- how much you sleep
- your body’s metabolism rate
The first thing influencing your calories burned while sleeping number is how much you weigh.
According to Harvard Medical, a 155 lb. person burns 46 calories per hour while sleeping, a 185 lb. person burns 56. So it looks like (roughly speaking) that for every 30 lb. of body weight, 10 calories per hour are burned while sleeping.
Use that info to find your calories burned while sleeping number.
Better sleep & metabolism increases overnight calorie burn
These two things also influence your body’s calorie burning ability, but they’re harder to put an exact “calories burned” number to:
Your sleep –
Fewer hours of quality sleep results in less calories per hour being burned.
Remember that study I mentioned earlier about 8 ½ hours of sleep vs. 5 ½?
Your metabolism –
Boosting your metabolism will result in burning more calories per hour than the average person at your weight.
There are several ways to boost your metabolism that science has proven.
I show how to put these metabolism boosters to work in your life in my article on weight loss after 60, posted here on heydayDo.
Wrapping Up
OK, time to hit the hay and burn a few hundred calories while I dream of dark chocolate and the smell of bacon on a Sunday morning 😁.
I hope this article on losing weight overnight is useful to you, and I wish you well on your fitness journey.
– greg
