Is it true that humans are taller in the morning than we are at night? If so, why?

There is a slight difference in height from the morning to evening but it may not be measurable. While the body is resting at night the discs take in water to replenish and nourish them.

Once we stand up in the morning the compressive force of the body weight causes the excess fluid to seep back out. The combined effect of muscle tension, gravity, posture, and reduced water content in the discs can make a small difference in height on a daily basis.

There is a much greater difference in height from young adulthood to old age. Thinning of the discs, degeneration of the spine, and the forces of gravity work over time to decrease the adult's height by as much as four to six inches. This difference may be more or less for some people.

Reference: 

Najma Farooq, MRCS, et al. Can Vertebroplasty Restore Normal Load-Bearing to Fractured Vertebrae? In Spine. August 1, 2005. Vol. 30. No. 15. Pp. 1723-1730.

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