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.
Meerman R, Brown AJ. When somebody loses weight, where does the fat go? BMJ 2014;349:g7257. doi:10.1136/bmj.g7257
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.
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.
Weissenberg S. Insensible water loss during sleep: a theoretical exercise. Adv Physiol Educ 2005;29(4):213–215. doi: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.
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
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). 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). 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). 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).
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 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, 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: 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.
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. Every study on this site, including the ones that cut against me, is on the science page.