According to an unparalleled new study on the personality types of donors:

[It] indicates that the sperm donors described themselves as being less worried, uncertain, shy and less subject to fatigue. Furthermore, we also found significant differences on the character dimensions, where the sperm donors showed higher means on self-directedness. This indicates that they perceived themselves as more autonomous individuals, with a capacity for responsibility, as behaving in a more goal-directed manner, and to be more resourceful and self-acceptant than the comparison group.

Vaughan Bell chuckles:

I suspect, however, that ‘not easily fatigued’ may be a selection bias due to the demands of the job.