Why your weight jumps overnight, and what a trend line does about it

You have a good week. You step on the scale on Saturday morning and it says you are a kilogram heavier than on Friday. Nothing about your week explains it, and the number on the screen is the only thing you can see.

That moment is where most people stop tracking. It is also, almost always, a moment where the scale is telling the truth about the wrong thing.

What the scale actually measures

A bathroom scale measures everything you are carrying at that instant: the water in your body, whatever is still being digested, what you drank before you stepped on. Several of those move by a lot from one day to the next, for reasons that have nothing to do with the change you are trying to see.

Water. Most of your body weight is water, and the amount your body holds shifts with salt, temperature, hormones, sleep and how much you drank. This is the biggest source of noise and the least predictable.
Stored carbohydrate. Your body stores carbohydrate together with water, so a day of more or less of it moves water with it.
What has not passed through yet. Food and drink weigh something until they don’t.
When you weigh in. Morning and evening readings on the same day can easily differ by more than a week of real change.

Put together, a swing of a kilogram or more between two mornings is ordinary. A real change of a few hundred grams a week is buried under it.

Why looking at one day at a time misleads you

If the real change is small and the daily noise is large, then any single reading is mostly noise. Comparing today with yesterday is comparing two noisy numbers, so the difference is noisier still. You can go up three days in a row during a week that, looked at properly, went down.

The fix is not to weigh in less often. It is to stop reading single days, and to read the line they make.

What a trend line does

Nala Tracker puts every reading on the chart as a faint dot, and draws a smooth line through them. The line is an exponentially weighted moving average, which sounds complicated and is not:

Each morning, the trend moves a small step towards the new reading — in Nala Tracker, once there are a few weeks of readings, about a tenth of the way. (In the first days it moves further, because there is less history to weigh against.)
Older readings never drop off the edge. Their influence fades gradually, a little more each day.

So a single high morning moves the trend by roughly a tenth of the jump, not the whole of it. If the next few days come back down, the line barely notices. If they stay up, the line follows them, because then it was not noise.

The app also measures your change per week from the trend over the last fourteen days, rather than from any two readings. That is the number to watch.

If you miss a day or a week, nothing breaks. The trend fades by the days that actually passed, not by the number of readings, so a gap does not make one reading count for more than it should.

Three habits that make the line clearer

None of these are rules. They just reduce the noise the trend has to smooth out.

Same time, same conditions. First thing after waking is the most repeatable moment for most people.
Same scale, on a hard floor. Carpet and a scale moved between rooms both add their own wobble.
Log the surprising days too. A high reading is information for the trend. Leaving it out makes the line look smoother than the truth.

The short version

Your scale is not lying. It is answering a different question from the one you are asking. The daily number answers “what do I weigh right now?” The trend answers “which way am I going?” — and that is the question worth tracking.