A unit of the limited energy a body has for a day. Not a metaphor for tiredness — a countable budget that runs out.
Old English spōn (a chip or splinter of wood), the shape later given to the eating tool; the energy-budget sense is a twenty-first-century coinage describing a day's limited capacity as a countable set carried and spent at a table.
A countable unit of a body's limited daily capacity, not a metaphor for feeling tired. The budget names itself as finite, so that running out is a fact rather than a failure.