Research

Why you weigh more on Monday: the weekly weight cycle

If Monday's weigh-in often looks like a step backwards and Friday's looks like a win, you are not imagining it. A large study that tracked people with connected scales for 18 months found a small but consistent weekly rhythm in body weight. Knowing about it makes a single weigh-in much easier to read.

What the researchers found

After removing each person's longer-term trend, so that a steady loss or gain would not hide a short cycle, the researchers looked at how weight varied by day of the week. The pattern was clear: weight rose over the weekend and fell back during the week. Across the group the within-week swing was about 0.35% of body weight. Weight was typically highest on Monday, followed by Sunday and Tuesday, and lowest on Friday.

To put 0.35% in everyday terms: for someone weighing 80 kg (about 176 lb), it works out to roughly 0.3 kg (well under a pound) between the top and bottom of the week. That is small, but it is the same size as a typical week of steady progress for many people — which is exactly why it can be so misleading when you look at one reading on its own.

The size of the swing was not the same for everyone. Men's weekly fluctuation was a little larger than women's (0.41% versus 0.29%), people aged 30–45 showed the largest swing and people over 60 the smallest, and the swing tended to be smaller at higher BMI. Participants in Portugal fluctuated the most of the three countries.

It matches an earlier finding

The result lines up with an earlier, smaller study by Orsama and colleagues (Obesity Facts, 2014), which analysed daily weigh-ins from 80 adults and also found weight rising at the weekend and falling through the week. That study suggested the weekday correction was most pronounced among people who were losing or maintaining weight. We looked at it in more detail in our article on weigh-in frequency and the weight trend line. The NoHoW study adds a much larger sample, automatic smart-scale readings and an 18-month window.

Holidays are different from weekends

The same paper looked at longer periods too. Over Christmas, weight rose by an average of 1.35%, and — unlike the weekend bump — it was not fully reversed in the following months: the authors report weight staying at least 0.35% above the pre-Christmas level. In other words, a weekly wobble tends to cancel itself out, while a holiday gain is more likely to stick. Our article on what an NIH study measured about holiday weight gain covers that side of the story.

What this means for reading your own numbers

The practical lesson is not about changing what you do on weekends; it is about how you interpret the data. A few general points follow from the study:

• ✓Compare like with like. A Monday reading compared with last Friday's mixes a weekly cycle into your result. Comparing Monday with last Monday, or week with week, removes most of it.
• ✓Watch the trend, not the point. Averaging or smoothing several days of weigh-ins flattens a repeating weekly pattern so the underlying direction shows through.
• ✓Give it more than a week. Because the cycle repeats every seven days, judging progress over several weeks is more informative than judging it day to day.

Limits of the study

The authors note several limitations. Everyone in the sample had recently lost weight and was enrolled in a weight-maintenance programme, so the pattern may differ in the general population. Whether people actually weighed in as often as asked could not be confirmed, some missing data had to be imputed for the longer-term analyses, and the seasonal results came from less than three years of data. The weekly finding is an association observed in this group, not a rule that applies to every individual.

Sources

1. Turicchi J, O'Driscoll R, Horgan G, Duarte C, Palmeira AL, Larsen SC, Heitmann BL, Stubbs J. Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention. PLoS ONE. 2020;15(4):e0232152. doi:10.1371/journal.pone.0232152 (full text on PubMed Central)
2. Orsama A-L, Mattila E, Ermes M, van Gils M, Wansink B, Korhonen I. Weight Rhythms: Weight Increases during Weekends and Decreases during Weekdays. Obesity Facts. 2014;7(1):36-47. doi:10.1159/000356147

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