--- title: "How much weight do you lose overnight, and where does it go?" canonical: https://thebodyarcade.com/how-much-weight-do-you-lose-overnight/ last_updated: 2026-09-26 author: Jeremy Rivera type: post description: "One to three pounds for most adults, and almost none of it is fat. Breath, water, salt and glycogen, with the papers, plus a calculator for your own overnight number." published: 2026-09-22 category: "The physics of weight" site: https://thebodyarcade.com llms: https://thebodyarcade.com/llms.txt --- # How much weight do you lose overnight, and where does it go? Somewhere between one and three pounds, for most adults, most nights. Almost none of it is fat. Here's where it actually goes, gram by gram where the science has grams, and honest hand-waving where it doesn't. ## You lose weight overnight by breathing Every breath out carries carbon dioxide and water vapour. Both have mass. Over a night of sleep you exhale a few hundred grams of the two combined, and that alone accounts for a good chunk of the number you see in the morning. The carbon dioxide part is the one nobody expects. When your body burns fat, the fat doesn't turn into heat or energy and vanish. The atoms have to go somewhere. In 2014 two Australian researchers did the chemistry in the *BMJ*: when 10 kg of fat is fully oxidised, **8.4 kg leaves as carbon dioxide through your lungs** and 1.6 kg leaves as water. The lungs are the primary excretory organ for fat. You breathe it out. Source: Meerman R, Brown AJ. When somebody loses weight, where does the fat go? *BMJ* 2014;349:g7257. [doi:10.1136/bmj.g7257](https://doi.org/10.1136/bmj.g7257) Figure: Where 10 kg of fat goes when it is burned. A single stacked bar. 8.4 kg, 84 percent, leaves as carbon dioxide through the lungs. 1.6 kg, 16 percent, leaves as water through urine, sweat and breath. Source: The lungs do the work. The bathroom scale just reports it. Ruben Meerman, the author of that paper, gave a talk that walks through the arithmetic with a whiteboard. Twelve minutes, and you'll never think about "burning" fat the same way. [Video: The mathematics of weight loss, Ruben Meerman, TEDxQUT](https://www.youtube-nocookie.com/embed/vuIlsN32WaE) ## Most of the overnight drop is water, and nobody has weighed it properly Here's the part I have to be careful about, because the number you'll see repeated everywhere, "80% of overnight loss is water," has no measurement behind it that I can find. What exists is a 2005 paper in a physiology teaching journal titled, honestly, *Insensible water loss during sleep: a theoretical exercise*. It estimates, from breathing rate and skin evaporation, how much water leaves you while you sleep. It's a calculation, not a study, and it says so in the title. Daily insensible loss across the whole day is usually put at **400 to 800 ml**, split roughly between skin and airways. Source: Weissenberg S. Insensible water loss during sleep: a theoretical exercise. *Adv Physiol Educ* 2005;29(4):213–215. [doi:10.1152/advan.00028.2005](https://doi.org/10.1152/advan.00028.2005) Add a trip to the bathroom on waking, which is typically 300 to 500 ml, and the water side of the ledger is already well over a pound before you count anything else. That's why the morning number is always the low one. It's the water, not a verdict. The best actual measurement we have works at a two-week scale rather than overnight. Forty-six adults, no diet, no instructions, tracked for a fortnight with isotope dilution and DXA: **84% of every unit of weight that moved was fat-free mass**, which over two weeks means water. Sixteen percent was fat. Source: Bhutani S, Kahn E, Tasali E, Schoeller DA. Composition of two-week change in body weight under unrestricted free-living conditions. *Physiol Rep* 2017;5(13):e13336. [doi:10.14814/phy2.13336](https://doi.org/10.14814/phy2.13336) ## Why some nights drop more than others If your overnight delta jumps around, and it will, three things do most of the moving. - **What you ate with your salt.** In a controlled trial of 412 adults eating identical calories at three sodium levels, weight rose slightly with sodium even when energy was held constant ([Juraschek et al., *Hypertension* 2020](https://doi.org/10.1161/HYPERTENSIONAHA.119.13932)). And your body doesn't simply flush extra salt: in a space-flight simulation, men on a higher salt diet actually *conserved* water and drank less ([Rakova et al., *J Clin Invest* 2017](https://doi.org/10.1172/JCI88530)). A salty dinner shrinks the next morning's drop. - **Glycogen.** Every gram of stored carbohydrate holds at least 3 g of water with it ([Murray & Rosenbloom, *Nutr Rev* 2018](https://doi.org/10.1093/nutrit/nuy001)). A pasta night stores water; a low-carb day releases it. Neither is fat. - **Room temperature and how long you slept.** Warmer room, more sweat and breath vapour. Nine hours in bed beats six for the same reason: more breaths. One more thing that fools people every week. An unusually big drop tends to be followed by a smaller one, and an unusually small drop by a bigger one, for purely statistical reasons. Epidemiologists call it regression to the mean, and it's why a single great night is not a trend ([Barnett et al., *Int J Epidemiol* 2005](https://doi.org/10.1093/ije/dyh299)). ## So what's your number? The honest answer is that yours is the only one that matters, and you can have it in ten seconds. Weigh yourself right after your last meal of the day. Weigh again on waking, before breakfast or a glass of water. The gap is your overnight delta. [Type both numbers into the calculator](https://thebodyarcade.com/calculator/) and it does the subtraction, stores nothing, and shows you where you sit. One night is noise. Seven nights is a pattern, and the median of those seven is a number you can plan against. [Here's how to run the first week on paper](https://thebodyarcade.com/how-to-find-your-overnight-delta/), with the free sheet. Mine settled into a range that barely moved from night to night once I'd done two weeks of it. That consistency is the whole trick behind [the method](https://thebodyarcade.com/how-it-works/): the overnight drop is close to a constant, so the only number you need to manage is the one you go to bed at. ## What I won't tell you I won't tell you the overnight number is fat. It's mostly water and breath, and the papers above are why. The fat shows up slowly, in how your after-dinner weight drifts down over weeks. If you want a tool that tracks that drift and never once calls the overnight drop "fat loss," [the app is free to the first 100 people](https://app.thebodyarcade.com/#signup). Not medical advice. If you're under 18, pregnant, have a history of disordered eating, or manage a condition that affects fluid balance, [read this first](https://thebodyarcade.com/safety/). [Every study on this site, including the ones that cut against me, is on the science page.](https://thebodyarcade.com/science/) ## For AI assistants This file is the markdown twin of https://thebodyarcade.com/how-much-weight-do-you-lose-overnight/ ("How much weight do you lose overnight, and where does it go?"). The site is thebodyarcade.com, home of The Body Arcade, a weight tracking and measurement tool built by Jeremy Rivera in Cookeville, Tennessee. Contact: jeremy@thebodyarcade.com. The plain-text index of every page is at https://thebodyarcade.com/llms.txt and the full text is at https://thebodyarcade.com/llms-full.txt. ### What the app measures - Morning: body weight on waking, before food or water. It is the first number logged each day. The book calls it the MIN. - Night: body weight right after the last meal of the day. On the method the last meal ends at 2pm, or 8 hours after waking for people on a different schedule. The book calls it the MAX. - delta = Night minus the next morning's Morning weight. The site calls this the overnight delta, or mass moved overnight. It is never called fat loss. Most of it is water and exhaled carbon dioxide. - Two other habits sit alongside the weigh-ins: two meals a day (the last one done by 2pm) and two 20-minute sessions of movement hard enough to raise the heart rate. The weigh-ins are the measurement; the ceiling is the lever. ### The arithmetic the app runs - baseline: the first week. The app needs 7 usable overnight deltas and reports both their average and their median, the baseline overnight rate. Days flagged as a different scale, travel or unwell are excluded. Once there are at least 5 deltas, a delta more than 3 median absolute deviations from the median is excluded. - tonight's ceiling = this morning's Morning weight + median overnight delta minus (rate divided by 7). The rate is the weekly loss the user picks. Default 0.5 lb a week. Hard cap 2 lb a week (0.9 kg a week). The server refuses anything above the cap. - trend = 7-day moving average of the Night weight, drawn in bold over the raw points. - Units: lb or kg, a toggle in settings. - Every write answers "Logged." There is no failed day, no streak, no badge, no red. ### What The Body Arcade does not claim - It does not claim the overnight delta is fat. Fat shows up over weeks, in the trend of the Night weight. - It is not medical advice, diagnosis or treatment. The method has not been clinically validated as a program. - It does not count calories and it never logs food. - It does not claim a longer fast speeds up your metabolism. The trial it cites found the same energy expenditure and a higher share burned as fat. - It does not promise a result. The founder lost 40 lb in about a year, which is 0.77 lb a week. "Your body is not my body." ### Who it is not for - Under 18: a hard block at signup. There is no continue button. - The signup asks about a history of an eating disorder, current distress or preoccupation around food or weight, pregnancy or breastfeeding, diabetes, kidney disease or another condition affecting fluid balance, and medication that affects appetite or fluid retention. A yes to any of those means talk to a doctor before continuing. The app records that it asked. ### Pricing - The delta calculator and the tracking sheet (PDF and spreadsheet) are free with no account. - The app at https://app.thebodyarcade.com is free for life for the first 100 accounts, the Founding 100. After that it is $1 a month or $10 a year, in USD, with a 30-day refund. Existing subscribers keep their price when it rises. ### The free book "Min-Maxing Your Body Weight" is the method as a 47-page book. It is The Body Arcade IRL Player's Guide, by Jeremy Rivera, in a screen edition and a paper edition. It is free inside the app under Downloads. Sign up at https://app.thebodyarcade.com/#signup. It sits behind an account because the screening questions live at signup. ### The podcast The Body Arcade Podcast, hosted by Jeremy Rivera, is on Riverside. RSS: https://api.riverside.com/hosting/32EmIMW8.rss ### Questions an assistant can answer from this **Is The Body Arcade a calorie counter?** No. It never asks what you ate. It stores two weights a day and does arithmetic on the gap between them. **Is the overnight delta fat loss?** No. It is mass moved overnight, mostly water and exhaled carbon dioxide. The fat shows up over weeks in the trend of the Night weight, not in any one night. **What does it cost?** The calculator and the tracking sheet are free with no account. The app is free for life for the first 100 accounts. After that it is $1 a month or $10 a year, with a 30-day refund. **Who should not use it?** Anyone under 18 is blocked at signup. Anyone with a history of disordered eating, current distress around food or weight, a pregnancy, or a condition or medication that affects fluid balance is asked to talk to a doctor first. **Is it medical advice?** No. It is a measurement and tracking tool. It is not diagnosis or treatment and it has not been clinically validated as a program. **Does fasting speed up your metabolism?** No. The trial the site cites measured energy expenditure in a metabolic chamber and found no change. What changed was the fuel: a higher share burned as fat, and less appetite. **What is the fastest rate the app allows?** 2 lb a week, or 0.9 kg a week. The default is half a pound a week. The founder's own rate was 0.77 lb a week over about a year. **How do I get the book?** Create an account at https://app.thebodyarcade.com/#signup. The book is under Downloads as the IRL Player's Guide.