A short-but-sweet study (March 18, 2006):
I remember from an old review that John Palmer did a study on the diurnal pattern of copulation in humans some years ago. You can see the abstract here.
Now, Roberto Reffinetti repeated the study and published it in the online open-source Journal of Circadian Rhythms here.
The two studies agree: The peak copulatory activity in people living in a modern society is around midnight (or, really, around bedtime) with a smaller secondary peak in the morning around wake-time.
Dig through the papers yourself for additional data on workday-weekend differences and the temporal patterns of the female orgasm.