Do you weigh more at night than in the morning?

Nothing mysterious is happening. A bathroom scale weighs everything that is currently part of you, and that includes the sandwich you ate an hour ago and the water still working its way through your system.

A litre of water weighs one kilogram. That is not an approximation, it is roughly how the kilogram was defined. So drinking a large glass of water and stepping straight back on the scale will show you half a kilogram heavier, immediately, before your body has done anything at all with it.

You are also losing mass all day through the bathroom, through sweat and through breathing. Intake and output do not run at the same rate, though. Most people take in more than they pass during waking hours, which is why the number drifts upward until bedtime and typically peaks in the evening.

None of it, and the arithmetic settles this rather than an opinion. Storing a kilogram of body fat requires eating roughly 7,700 kcal more than you burned. To genuinely gain a kilogram of fat between breakfast and bedtime you would have to eat about 7,700 kcal above maintenance in a single day, on top of everything you normally eat.

A large, indulgent day might put you 1,000 to 1,500 kcal above maintenance. As fat, that is around 130 to 195 grams. Meanwhile the scale may be showing you two kilograms heavier. The gap between those two figures is food, drink and water sitting inside you, and it is leaving again shortly.

Three routes, running all night while you do nothing:

• Urine. The single largest overnight loss, and the reason the first trip to the bathroom changes the reading noticeably.
• Breathing. Exhaled air carries water vapour. You lose a surprising amount of mass simply by breathing for eight hours, which is also how the carbon from burned fat physically leaves your body.
• Sweat. Even in a cool room, and more in a warm one.

That is why a morning weight is lower than the previous evening without anything meaningful having happened. It is also why sleeping in a hot room can make you look like you lost half a kilogram overnight. You lost water, and you will drink it back before lunch.

First thing in the morning, after using the bathroom, before eating or drinking. Not because that number is more true than any other, but because it is the most repeatable point in the day. Everything after it depends on when you last ate, drank or used the bathroom.

What matters far more than the time is that it is always the same time. Comparing a Tuesday morning to a Friday evening tells you nothing except that you ate dinner on Friday.

And once you are weighing consistently, read the weekly average rather than any single reading. Day to day the number still moves for reasons that have nothing to do with fat, which is covered in why your weight changes day to day.

Stop weighing yourself in the evening and being disappointed. You are measuring your dinner.

It also explains a specific trap. People start a calorie deficit, weigh themselves at a random moment a few days in, see a number higher than the one they remember, and conclude the deficit is not working. Usually they have compared a morning reading to an evening one, or a fasted reading to a post-meal one. The deficit was fine. The measurement was not.