Are there any circumstances that allow for a couple to be intimate during the day time (i.e. wanting to be together on shabbos but being too tired to do so on friday night)?
The Gemara writes that one may not have marital relations during the day (Niddah 17), and therefore the night, and the more so Friday night, is the best option.
Yet, where circumstances don’t allow for this, it is permitted to have marital relations in the day, provided the room is darkened (Niddah loc. cit.; Shulchan Aruch 240:11).
According to the Magen Avraham (25), this is permitted even on a lechatchilah level. But many write that it is only permitted under extenuating circumstances (see Benei Zion 240:14; Iggros Moshe, Even Ha-Ezer 1:102 writes that it is only permitted in times of a great need).
Authorities that cite from Kabbalah sources are still more stringent in this matter (though there is some dispute concerning Shabbos day).
Of course, one must ensure not only that the room is dark, but also that there is no chance that somebody will disturb.