Many are stringent: if they become a bal keri (have a nocturnal emission) for any reason they don't learn or pray before going to the mikveh ("tevilas ezra"). However, even according to those that are stringent in this (chassidim, etc.) is there any possibility of leniency in order to avoid missing out on a certain regular Torah study one does each night (that he wasn't able to study before having relations or that he simply wants to stay up and study afterwards and he can't get to the mikveh that night)?
I have heard the Yismach Moshe is lenient in such a case but I was never able to locate the leniency in that book. Perhaps someone here knows that citation or another. Obviously, if one would miss out on doing a mitzva at its right time, he would be lenient; I'm asking more in a general case when one merely wishes to learn at night and can't get to the mikveh.