It is a fact that a person's height is greater in the morning compared to the evening. Although this difference may be subtle in daily life, it can reach several millimeters and, in certain individuals, exceed one centimeter, being measurable with a tape measure placed against a wall.
The reason for this variation lies in the spine. The intervertebral discs, which are cartilaginous structures located between the vertebrae and act as shock absorbers, lose fluid over the hours due to body weight and the influence of gravity.
When standing or sitting, the spine continuously supports the weight of the head, torso, and arms. This continuous pressure gradually compresses the discs, which have a water-rich gelatinous core. When lying down at night, this pressure ceases, allowing the core to recover some of the lost fluid and restore the original height until dawn.
The spine is composed of 23 intervertebral discs, distributed from the neck to the lumbar region. Each disc has two distinct parts: a more rigid outer ring called the annulus fibrosus, and an inner gelatinous core known as the nucleus pulposus, which is abundant in water and proteoglycans—molecules that act as water attractants.
This core behaves like a sponge under pressure. While the body is upright or seated, the weight causes the vertebrae to press against each other, compressing the discs and forcing some water out. This accumulated compression throughout the entire spine is enough to decrease a person's total height during the day.
At night, in the absence of body weight exerting pressure on the spine, the process reverses: the proteoglycans in the nucleus pulposus begin to attract water from surrounding tissues again, and the discs recover some of the volume they had lost. After a night's sleep, the height tends to return to the closest level to its maximum.
Additional information
The same mechanism explains why astronauts are taller during their space missions. In the absence of Earth's gravity compressing the spine, the intervertebral discs absorb more water than usual and remain expanded for several weeks.
Studies conducted by NASA have documented that astronauts became several centimeters taller after long periods in microgravity, an effect that reverses shortly after returning to Earth, when gravity resumes its compression on the spine.
On Earth, this variation can be observed without the need for complex equipment. Simply measure your height by leaning against a wall as soon as you wake up and repeat the measurement before going to sleep. The difference is usually small but noticeable—and tends to be greater in people who spend the day standing or carrying weight, compared to those who remain seated most of the time.
Individuals who practice stretching or spend time lying down during the day also tend to notice a faster recovery of height, indicating that the spine, like any shock absorber, needs rest to return to its ideal shape.