Losing weight overnight?

Woke up yesterday at 225 lbs. Went to bed at 227. Woke up at 225.

Where did this weight go and how did I lose it?

I don’t know about you, but the first thing I absolutely have to do when I wake up is pee. So I’d say it went down the toilet. And some of it evaporated as sweat.

You can easily perspire and exhale one pound of water, not to mention other ways to get rid of it. That alone will be within the margin of error of your scale if it doesn’t cover the entire two pounds.

To make clear when fat is broken down the waste is exhaled as carbon dioxide. Real weight loss, where calories in is less than calories out, as opposed to just a temporary water loss which you gain back during the day, is mostly exhaled as carbon dioxide.

And of course the average male urinates from 800ml - 2000ml a day. Which means your weight will fluctuate easily 1 kg = 2 pounds during the day depending on how full your bladder is.

Poop, pee, water retention.

It would be a much more serious concern if you didn’t “float” weight at night. That would typically be indicative of extreme malnourishment and dehydration. In my wrestling days, I routinely dropped nearly 1/3 of my body weight. When I would get close to my target weight, I was unable to sweat - that’s not a good thing when you’re in training; heat exhaustion can kill you.

In extreme deprivation, the body gets very selfish. It knows it needs liquid, sugar, and electrolytes (the brain basically lives on sugar and water, and needs to perform electrical functions). I recall a teammate who had “sucked” too much weight before a tournament; in between matches, we had to hold him down (he was crippled by cramps) and force him to suck on those awful stadium nachos to get salt into him. But he wasn’t allowed to swallow… still had to make weight for the next session.

As noted up-thread, respiration dumps a lot more fluid than most people realize. Especially if you’re sleeping with your mouth open. It’s no simple irony that most people wake up wanting both to urinate and drink something - the stuff leaving your bladder has been segregated (because it’s toxic), and your body wants (demands!) that you replace it.

Also, unless you have a humidifier in your bedroom, the winter air tends to be very dry. That speeds up the evaporation process while you are sleeping (you are inhaling dry air and exhaling moisture). That, plus sweat and urination, results in a lot of water loss overnight.

Or else your scale is inaccurate.


I am regularly a couple pounds lighter in the morning than the evening. Water loss, metabolism, it all goes somewhere. Then the next night, I am back up.

Digital scale pretty accurate. Not from the Office of Weights and Measures, but accurate enough. I guess I was one of those people who didn’t realize that one exhales so much moisture. It was just odd. Thanks for the replies. Looking to drop another 20. Better take a Unisom and hit the rack.

Go put some 5 and 10 kg weights on your digital scale at different times of day, humidity, different temperatures etc. You’ll soon see its probably only accurate to within .5 kg / 1 pound. Which is perfectly reasonable, since the average person doesn’t need to know his weight any more accurately than that.

Exhalation can’t be that much. For a typical adult, the resting respiration rate is about 15 breaths per minute, and the tidal volume = 0.5 liters. For eight hours, that’s 3600 liters of respired volume, or 3.6 cubic meters.

Assume you inhale perfectly dry air. Your lungs will warm that air up to body temp (37C), and humidify it to 100% relative humidity, which means water vapor will be present in your exhaled breath at a density of 0.0439 kg/m[sup]3[/sup].

I came up with a total exhaled water mass of 0.159 kg (0.35 pounds). The remainder of the OP’s 2-pound overnight loss will be due to perspiration, defecation or urination.

You’re missing something. We breathe in O2 but we breathe out CO2. We are losing mass from the carbon molecules that get attached to our waste in every breath.

This is a bit of a “wow, I never thought of it that way” moment for me. I presume the amount of weight we lose by radiating heat is minuscule (it falls under E=Mc^2). Heavy perspiration would add a bit more through chemicals (mostly salt) that are perspired. What about urination/defecation? Is some of the by-product of burning fat lost that way? I guess what I want to know is, for an average pound of weight loss, how much of it was expired away?

OK, let’s see. According to this site, exhaled air is 5-6% CO[sub]2[/sub]. Call it 6%. From my math earlier, 3.6 m[sup]3[/sup] of exhaled air per night, which means 0.216 m[sup]3[/sup] of pure CO[sub]2[/sub]. Density of CO[sub]2[/sub] is 1.977 kg/m[sup]3[/sup], so 0.427 kg of CO[sub]2[/sub], so 0.116 kg (0.256 lb) of carbon.

Between that and the 0.35 lb of water leaving via respiratory humidification, we’ve now accounted for about 0.6 lb of nocturnal weight loss. Still looking for another 1.4 pounds.

If you’ve ever slept in a car or a sealed waterproof tent, you’d realize how much water you sweat and breathe out. I don’t have any numbers, but the enclosed space often becomes noticeably humid.

Piss, sweat and inaccurate scales. I went through a stage of weighing myself several times a day when I started on a diet / exercise program recently. Same as OP I always weighed the least first thing in the morning after my morning urination. Also, after an intense work out session especially high intensity interval training, you can weigh 2 pounds / 1 kilo less after one hour. Of course its mostly water loss from perspiration so you gain it back after rehydration. Normal daily weight fluctuations can be up to 5 pounds / 2.5 kg.

On this kind of training reginmen, the real weight loss will be gradual over the day. It raises your metabolism for roughly 14 hours after you do it, so you’ll be breathing out more CO[sub]2[/sub] than normal for that period.