Why Your Weight Jumps Two Pounds Overnight: Kevin's Diet and the Bathroom Scale
The short version: your weight on any one morning is your real weight plus noise from water, salt, food and the bathroom. If that day-to-day noise has a typical size of 1.3 pounds, two mornings differ by more than 2 pounds about 28% of the time with no change at all. Reacting to each morning is reacting to noise. Average the week, or look at a 7-day moving average, and the trend shows through.
Cousin Kevin has six weeks until his sister's wedding and a suit that was bought for a slimmer Kevin. He has started a diet and weighs himself every morning, then posts the result in the family group chat with a commentary.
Monday: down 2.4 pounds. "THE PLAN IS WORKING." Tuesday: up 2 pounds. "Sabotage. Someone put salt in something." Wednesday: down 0.4 pounds. "Stable. Holding the line." By Friday, the group chat had muted him, and Aunt Linda had started replying only with a picture of a weather vane.
Your weight has weather
The number on the scale isn't just body fat. It includes the water you're holding, the food still in you, the salt you ate yesterday, whether you've been to the bathroom and the exact time you stood on the scale. Most of those change from day to day by amounts that dwarf a week of honest dieting.
Let's say Kevin's true weight is steady and his daily noise has a standard deviation of 1.3 pounds: on a typical day the scale is within about 1.3 pounds of the truth, and occasionally further. That's a fairly ordinary amount of wobble for an adult.
How big a jump is just noise?
- The difference between two mornings has a typical size of about 1.8 pounds.
- The two mornings differ by more than 2 pounds about 28% of the time, with no real change at all.
- Meanwhile, a solid diet might take off 1 pound in a week: less than one day's noise.
So Kevin's triumphs and disasters are mostly weather. Reading meaning into each morning is like checking whether summer has arrived by looking at one cloud.
Chance two mornings differ by more than this, with no real change
How to see the trend
The fix is to stop looking at single days. Average them. A week of noisy mornings averaged together cancels much of the noise: up days and down days pull against each other, and what's left is mostly the real change. A moving average does this continuously: each day, average the last seven mornings. The line is smooth, slow and boring, which is exactly what makes it honest.
Kevin's daily numbers looked like a heart monitor. His seven-day average fell by about a pound a week, steadily, for five weeks. The diet had been working all along. The daily numbers had been too noisy to say so.
How to read any noisy number
1. Learn the size of the wobble. Weigh yourself for two weeks without changing anything. The range you see is your noise.
2. Ignore changes smaller than the wobble. Two pounds up or down in a day is weather, not climate.
3. Judge the average, not the latest. A weekly average or a 7-day moving average shows the trend. One morning shows the morning.
Where it costs you money
Daily share prices are bathroom scales. A stock can move 2% in a day for no reason anyone will ever find, and checking your portfolio every morning turns that noise into a series of triumphs and disasters. Investors who check less often tend to trade less and react less to noise. Look at the long average, and at whether your reasons for owning something have changed, not at this morning's weather.
The moral
Kevin switched to posting a weekly average in the group chat. The group unmuted him. On the morning of the wedding, he weighed himself out of habit and was up 2.6 pounds, from the curry the night before. He wore the suit anyway. It fitted, because suits, like averages, don't care what happened yesterday evening. Linda sent the weather vane one last time, with a heart.
The 1.3-pound day-to-day noise is an illustrative figure; real daily variation differs between people. Educational content, not financial advice.