Twenty-three discs
The adult spine has 6 cervical, 12 thoracic and 5 lumbar discs. Together they make up about 25%–33% of spinal length.
Stature · water · load
Nineteen millimetres by midnight 1.1% of stature in the 1984 study
In a controlled 24-hour measurement, eight adult men lost a mean 19.3 mm of stature between waking and midnight. More than half of the change occurred within the first hour of getting up.
The number has clear boundaries. The sample was small and all male. Another study of 55 young adults measured a lower mean loss of 9.4 ± 3.12 mm. Daily stature varies across people and protocols.
The mechanism
A water-seeking column
The adult spine has 23 intervertebral discs. Bones keep the same length throughout the day, while the soft, water-rich tissues between them respond slowly to load. The change in stature is the visible trace of fluid movement and posture.
A core containing 66%–86% water
The nucleus pulposus contains aggrecan. Fixed negative charges on this molecule attract water and create osmotic swelling pressure.
Load shifts the balance
While upright and moving, hydrostatic pressure gradually drives water out of the disc. During recumbent rest, the osmotic gradient favours water returning and height recovering.
The evidence
Same direction, different amplitudes
The studies should not be pooled into one mean. Their samples, times and instruments differ. Together they show that the phenomenon is real and that one number cannot predict an individual's value.
| 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 |
Your instrument
Measure the same person twice
A small change requires repeatable positioning. Use the same place, the same position and three readings at each end of the day.
- Choose a stable reference. Use a hard floor, straight wall, bare feet and the same hardback book.
- Repeat the position. 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.
- Take three readings. Breathe in deeply, place the book firmly on the crown, mark the wall, then step away and reset your position each time.
- Keep the ends of the day. Take the first set after waking and the second before bed. Note the times and any unusual activity.
The protocol adapts the NHANES standing-height position. The studies use calibrated stadiometers; the wall-and-book method estimates the difference at home.
Limits and method
A measurement with a time and context
Morning stature and evening stature answer the same question at different times. For comparisons across days or people, the time and protocol belong to the value.
What the evidence supports
Anthropometry studies find a measurable daily decrease, while MRI shows concurrent changes in disc water and height. Osmotic pressure and axial loading provide the physical mechanism.
What remains open
One pair of measurements cannot describe spinal health. Activity, sleep, age, positioning and the instrument can change the result. This page provides no clinical thresholds and does not turn variation into a diagnosis.
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.
Sources
Where the values come from
Reported values retain the population and time window of each study. The time model uses the points published by the 1984 study and labels the interpolation between them.
- Reilly, Tyrrell and Troup, Circadian variation in human stature, 1984 24 hours, 8 adult men, 19.3 mm, and the pace of loss and recovery.
- Chaparro and colleagues, Diurnal Variation of Height in a Colombian Young Adult Population, 2014 55 young adults, 9.4 ± 3.12 mm and the distribution of change across time windows.
- Zhu and colleagues, Segmental Quantitative MR Imaging Analysis of Diurnal Variation of Water Content in the Lumbar Intervertebral Discs, 2015 Morning and evening MRI in 20 healthy volunteers, with regional analysis of lumbar discs.
- NCBI Bookshelf, Anatomy, Back, Intervertebral Discs, updated 2026 Anatomy of 23 discs, their share of the spine, water content and osmotic mechanics.
- CDC/NCHS, NHANES Anthropometry Procedures Manual, 2021 The standard position for measuring standing height.