Some days are better than others. That goes with many things, including creativity and inspiration. Environmental effects are surely important, but what about inherent causes? I leave the speculations there, but encourage you to have a look on the paper below and make your own thinking and conclusions.
Hagemann G, Ugur T, Schleussner E, Mentzel H-J, Fitzek C, et al. (2011) Changes in brain size during the menstrual cycle. PLoS ONE 6(2): e14655. doi:10.1371/journal.pone.0014655