There is a fire-fly in the southern clime
Which shineth only when upon the wing;
So it is with the mind: when once we rest,
We darken.

Philip James Bailey (1816-1902) English poet, lawyer

Festus, Sc. “A Village Feast – Evening” [Festus] (1839)
    (Source)

Usually paraphrased (earliest source (1872)):

The firefly only shines when on the wing.
So is it with the mind -- when once we rest
We darken.