Energy and vitality course through my veins.

Lying completely still, you are moving about five litres of blood a minute into every corner of yourself, and none of it stops on the days you feel flat.

Vitality gets pictured as bounding up a hill. Most of it is this instead: chemistry running at full tilt underneath a body that appears to be doing nothing at all.

Which is worth knowing on the mornings you cannot feel any of it. Feeling energetic and being alive are two different measurements, and only one of them is taken by mood.