ageing

You are taller in the morning than you are at night. This is not a metaphor. It is measurable, and the mechanism is your spine. Gravity compresses your intervertebral discs throughout the day. By evening you are shorter than you were at breakfast. Overnight your discs rehydrate and you regain the height. Over a lifetime this process is less reversible. That is why your grandparent is shorter than they used to be.