The method

Why the daily scale lies

The scale is honest about this moment and noisy about the week. A single reading includes everything in your body right then — water, and food and drink still being digested — so two mornings in a row can disagree without anything meaningful having changed.

What moves the number from day to day

Researchers who track daily weights see regular ups and downs. In one study of people weighing themselves daily, weight tended to rise over weekends and fall through the week^[1]; a larger study measured within-week swings of about 0.35% of body weight on average^[3].

When you weigh matters too. In a small study of older hospital patients, readings taken at different times of day varied by roughly 1–4% over three days, while readings taken at the same time each day moved 0.4 kg or less^[2].

Water is part of it. The body stores carbohydrate as glycogen together with about three to four times its weight in water, so changes in stored glycogen move water — and the scale — with them^[4].

What a trend line does

Light Years' trend line is a weighted average that leans on your recent weigh-ins and smooths the day-to-day jitter. Drawn under your raw dots, it shows the direction your own logs are heading. When one reading jumps, the line moves only a little.

Why the line trails your scale

While your readings are falling, the trend runs a little behind your lowest mornings — a weighted average has to see several readings before it moves. When your readings level off, the line catches up. If your scale says five pounds down and Light Years says three, that is the averaging at work, not a different measurement.

What it can't tell you

The trend describes the numbers you logged — nothing more. It can't tell you why your weight changed, what changed underneath (water, muscle or fat), or whether a medication is working. Gaps between weigh-ins make it less certain: with few recent readings, one unusual morning carries more weight.

How to read it

Weigh whenever suits you; the same time of day and similar conditions make the dots tidier, and the line forgives the rest. Watch the line, not the dot.

This guide is about measurement, not treatment. It is general information, not medical advice.

Light Years draws this smoothed line from your own weigh-ins, on your device. Open my tracker or browse the other guides.