A1: We are all indeed taller in the morning. There are two components to this. In a growing child, the growth hormone is secreted in pulses overnight. This acts through several intermediary steps to cause lengthening of the bones at the end-plates (epiphyses). Accurate measurements of the forearm or lower leg using specialised apparatus or X-rays can record this night-time growth.
The most marked effect, however, which occurs even after growth has ceased, is caused by postural compression of the spine under the effect of gravity. This was recognised in mediaeval French folklore: it was common practice to apply for recruitment to the army first thing in the morning in order to appear taller. In 1724, Reverend Joseph Wasse from Aynho in Northamptonshire, U.K., measured a loss of up to 15 mm through the day. This was more marked in young and active labourers.
A2: Another factor concerns the inherent curvatures of the spinal column. This has a convexity backwards in the thoracic or chest region, called a kyphosis, and a concavity in the lumbar region or base of the back called a lordosis. These curves vary with body weight and position. As a result, the spinal column tends to press downwards when in an upright position, altering these curvatures, and hence shortening the spinal length. When lying down, the reverse happens and the column lengthens again.
The combination of disc and curvature factors causes height change and is about 16 mm during the course of the day in the average adult.
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