# Nineteen Millimetres by Midnight

In a controlled 24-hour measurement, eight adult men lost a mean 19.3 mm of stature between waking and midnight. The peak was measured at 07:30 and the trough at 00:00; more than half of the change occurred within the first hour of getting up and 80% within the first 3 hours.

The figure describes a small, all-male sample. A separate study of 55 young adults measured a lower mean loss of 9.4 ± 3.12 mm. The difference between the studies matters: daily stature varies across people, activities and protocols.

[Ediția în română](https://mariuscomper.uk/nouasprezece-milimetri/)

## The mechanism

The adult spine has 23 intervertebral discs: 6 cervical, 12 thoracic and 5 lumbar. Together they make up about 25%–33% of spinal length.

The nucleus pulposus contains 66%–86% water and negatively charged aggrecan molecules. Aggrecan attracts water and creates osmotic pressure. While upright and moving, hydrostatic pressure gradually drives water out of the disc; during recumbent rest, the osmotic gradient favours its return.

## Three windows onto the same day

| Study | Participants | Window | Result |
| --- | ---: | --- | --- |
| Reilly, Tyrrell and Troup, 1984 | 8 adult men | 9 readings over 24 hours | 19.3 mm, or 1.1% of stature; over 50% in the first hour and 80% in the first 3 hours |
| Chaparro and colleagues, 2014 | 55 young adults | 4 readings between 06:00 and 18:00 | 9.4 ± 3.12 mm on average; 60% of the change between 06:00 and 10:00 |
| Zhu and colleagues, 2015 | 20 healthy volunteers | Lumbar MRI in the morning and evening | Significant T₂ signal decrease in the nucleus pulposus and a 2.1% decrease in measured L1–S1 length |

T₂ signal is sensitive to tissue composition and mobile water. The MRI result supports water redistribution without turning every millimetre of stature into a direct disc measurement.

The illustrative day model uses the points reported in 1984: 0% on waking, over 50% after the first hour, 80% after 3 hours and 100% at midnight. At the 3-hour mark, 80% of the 19.3 mm mean is 15.4 mm.

## Measure your day

1. Use a hard floor, straight wall, bare feet and the same hardback book.
2. Keep heels together, feet flat, shoulders relaxed and eyes forward. The line from the ear canal to the lower edge of the eye should be parallel to the floor.
3. Breathe in deeply, place the book firmly on the crown and take three readings, resetting your position each time.
4. Repeat the session in the evening, in the same place and position.

Calculate the median of the three morning readings and the median of the three evening readings. The observed loss in millimetres is `(morning median in cm − evening median in cm) × 10`. The percentage of morning stature is `loss in mm ÷ morning stature in mm × 100`.

Dividing the result by 23 gives an easy-to-picture even-share value. Discs differ in size and load, while posture also contributes to the total change. The result does not assess spinal health and has no diagnostic value.

As measurement safeguards, the instrument treats a difference below 3 mm as too small for home interpretation, asks for a recheck above 30 mm, and flags sets whose readings differ by more than 5 mm. These are cautious quality filters with no clinical meaning.

## Limits

Time, activity, sleep, age, positioning and the instrument can change the result. The studies use calibrated stadiometers; the wall-and-book method estimates the difference at home. For comparisons across days or people, the time and protocol belong to the value.

## Sources

- [Reilly, Tyrrell and Troup, *Circadian variation in human stature*, 1984](https://pubmed.ncbi.nlm.nih.gov/6600017/)
- [Chaparro and colleagues, *Diurnal Variation of Height in a Colombian Young Adult Population*, 2014](https://doi.org/10.4067/S0717-95022014000400038)
- [Zhu and colleagues, *Segmental Quantitative MR Imaging Analysis of Diurnal Variation of Water Content in the Lumbar Intervertebral Discs*, 2015](https://doi.org/10.3348/kjr.2015.16.1.139)
- [NCBI Bookshelf, *Anatomy, Back, Intervertebral Discs*, updated 2026](https://www.ncbi.nlm.nih.gov/books/NBK470583/)
- [CDC/NCHS, *NHANES Anthropometry Procedures Manual*, 2021](https://wwwn.cdc.gov/nchs/data/nhanes/public/2021/manuals/2021-Anthropometry-Procedures-Manual-508.pdf)

A project by [Marius Comper](https://mariuscomper.uk/en/) about normal stature physiology, published 27 August 2026.