For most photographic materials, reciprocity is valid with good accuracy over a range of values of exposure duration, but becomes increasingly inaccurate as we depart from this range: this is reciprocity failure. As the light level decreases out of the reciprocity range, the increase in duration, and hence of total exposure, required to produce an equivalent response becomes higher than the formula states; for instance, at half of the light required for a normal exposure, the duration must be more than doubled for the same result. (wikipedia.org)
I've been really preoccupied by my studies at The Royal Academy of Art in The Hague, Netherlands. Things can get a bit rough, and sometimes more exposure doesn't mean more inspiration...
(...which came, nevertheless, mainly from Caravaggio!)