Understanding personal, daily bio-rhythms is a big part of the writing process -- are you a morning writer, a late-night writer, or maybe your best time is after a siesta? But don't ignore the impact of season on your motivation to write and even the quality of your prose.
Northern writers with four distinct seasons to deal with have a more complicated writing life, I think, than someone who lives, say, in San Diego or Florida or Arizona. The weather, the temperature, the light, the length of day is always changing, and with it my mood. My inclination to write competes with other, often sudden opportunities (or necessities) to be outside: to plunge into the river on rare hot day in northern Minnesota; to shoulder my shotgun and head to the woods on beautiful October afternoon; to shovel the roof in January.
Spring in Minnesota is my most difficult time to write. I am hostage to the crazy, mad arrival of returning birds and their mating call and flutter--not to say ice-out on the Mississippi, then the peepers and various frog songs, and turtles to watch out for on the highways. I limp along with my writing, doing what I can (usually well before dawn) until the urge to be outside and do something (something "real", that is) overtakes me. Seasonal affective disorder is not just about being light-starved and gloomy; it makes me light-drunk, manic and attention-deficit-disorderd when it comes to writing.
My point here: if you are struggling to write because of short days of winter, or long crazy days of summer, or other seasonal factors (allergies!) try as best you can to move your "ball" (your novel) forward at least in some way. If it can't be several pages of good prose, it can be notes on characters; or sketches of chapters-to-come; or skimming in a novel that has prose like you'd like to have some day; or certainly editing, tweaking, or tinkering (but not too much) of what pages you do have.
Some progress, every day, is the goal. It is what will eventually bring you to that exhilarating last page, which is when the real work of revision begins. But take pleasure in getting to this point–a completed draft of your novel–because most aspiring writers never get there.
And recognizing all of the obstacles against the act of writing--including our latitude and longitude--is no small part of the writer's life.