When engaged in a project full of invention and discovery I continue as evening falls, until the studio darkens and neither lamplight nor overhead lighting are sufficient. (None rival daylight.) Then I put the project respectfully aside eager for sunlight's return and the opportunity it brings to return to making.
Have had to coax myself from the studio on many occasions like a parent reasoning with a child who is still romping about the playground at dusk. The child wants to linger. "We can come back tomorrow," the parent reassures her. Frequently I've tidied up the studio at night with the same promise to myself: you can come back tomorrow!