My favorite writer on this, as on many things writing, is Robert Boice. In How Writers Journey to Comfort and Fluency: A Psychological Adventure, he writes about fatigue:
...Fatigue makes writing, now and later, less desirable; it can trigger impatience as productivity lags and as the tempation grows to pick up the pace; it can bring anxiety and its pernicious narrowing; and it heightens susceptibility to irrationality.
...Our attention wanders and we fight back, much like a sleepy driver trying to stay awake and on the road. Next, commonly, we come to sudden doubts about what we are doing; we begin to question the purpose and worth of the writing.(1) Then with equal impulsivity, dysphoria takes hold and with it intrusive thoughts and an inability to suppress them.(2) With that comes a reinstatement of mindlessness in moving to quick, relieving solutions such as putting off writing until a "more propitious time." What happens at worst? Writers settle into the interiorization of self-focus and into reclusion. (p. 225)
(1) Daly J.A. (1985). Writing apprehension". In M. Rose (ed.), When a writer can't write, pp. 43-82. New York: Guilford.
(2) Conway M., Howell A. & Gainnopoulos C. (1991). "Dysphoria and thought suppression". Cognitive Therapy and Research, 15, 153-156.