We are told by those who should know that people work better during certain stages of their circadian rhythm, normally when they reach their greatest core body temperature and corresponding optimum level of brain wave activity. For writers, this equates to clearer thinking and greater creativity during these peak times.
When this period occurs is dependent on the sleep-wake cycle for each person, however, something that’s controlled by exposure to both natural and artificial light. The lowest level of body temperature and brain wave activity occurs, on average, two hours before a person wakes. The highest level, then, is approximately twelve hours later. For instance, if you wake naturally at 6 a.m., then 4 a.m. should be your low period and 4 p.m. your high. Writing from 2 p.m. until 6 p.m. would then take you through your optimum work period. In any case, writing while your core temperature is rising is preferable to writing when it is falling. It’s not practical to write only at the top of your productive cycle, of course, but the concept is something to think about when you run into difficulty with a story.
Added to this theory is another which says periods of greater creativity can also be associated with the light-dark cycles of the calendar year. After years of monitoring my output level, I’ve noticed that writing is easier after the winter solstice, as daylight hours slowly increase, and it’s most difficult in the late summer and fall as darkness arrives earlier. Or maybe it’s just that not much is happening during January and February, so it’s easier to concentrate!