It is common to notice that a person's height is slightly greater upon waking compared to the time before going to sleep. This difference can reach about 1% of the total height throughout the day, corresponding to approximately one or two centimeters. This phenomenon is primarily related to the spine and not to bone shrinkage.
Between the vertebrae are the intervertebral discs, structures that act as shock absorbers, aiding in the distribution of body weight and absorbing impacts. Throughout the day, activities such as walking and standing intensify pressure on these discs, causing their compression and temporary change in volume.
When lying down to sleep, the spine no longer supports the body weight in the same way. In this state, the discs are able to recover some of the volume they had lost, resulting in an increase in height. This is a physiological process that occurs daily.
Scientific studies confirm height variation
This phenomenon has been documented through imaging tests. A study conducted in 1994, published in the scientific journal Spine, monitored eight healthy men using magnetic resonance imaging. The researchers examined three lumbar region discs and found a noticeable decrease in volume, height, and diameter after periods of activity, which helps explain the fluctuation of stature throughout the day.
It is important to note that this variation does not imply that everyone loses exactly one or two centimeters; the degree of change depends on individual factors, level of physical activity, and the methodology used to measure height. Furthermore, it is not a permanent shortening, as bones maintain their constant size; the modification occurs mainly due to spinal compression under load.
The effect of microgravity
The impact of this effect becomes even more evident when gravity is drastically reduced. In conditions of microgravity, such as those found in space flights, the body is subjected to much less load than on Earth, which prevents the usual compression of the intervertebral discs.
With less pressure, the discs can expand and the spine can lengthen, allowing astronauts to achieve greater heights during extended missions. A 2016 study, also published in the journal Spine, tracked six NASA astronauts before and after a six-month mission on the International Space Station. The research analyzed changes in the spine's supporting muscles and the intervertebral discs themselves.
The article mentioned an approximate increase of five centimeters in body height as a change linked to prolonged stay in microgravity, although this data was presented as established knowledge in the literature, rather than as the main finding from the measurements taken with the six astronauts.
However, being taller in space does not guarantee comfort; astronauts can experience back pain during space missions, and prolonged exposure to microgravity causes various muscular and bone modifications.
Adaptation upon return to Earth
Returning to Earth also requires a period of readaptation. As soon as gravity returns to its normal function, the spine begins to bear load again. NASA investigates issues related to intervertebral discs after long missions, pointing out that any potential injuries have multifactorial causes.
Therefore, it is incorrect to state that the mere act of lengthening the spine in space causes injuries; the lengthening occurs concurrently with a series of other transformations induced by microgravity.
In the terrestrial environment, a much more subtle version of this process can be observed. One only needs to compare the height measured without shoes immediately upon waking with a measurement taken before sleeping, using the same location and maintaining the same posture. If there is a difference, it may vary from a few millimeters to over one centimeter, reflecting the spine's response to the load accumulated during the day, and not necessarily a measurement error.