Lately, when this happens, one of the contributing factors is simply discouragement over the "non-restorative sleep" that is sometimes a feature of the chronic illness I live with. That's the medical term for diligently going to bed at a reasonable time, sleeping soundly for eight or nine hours and waking up as tired as when you went to bed. In a kind of adolescent shoot-myself-in-the-foot-way a week of this can inspire me to think, "What the heck- may as well have some fun since I am going to be exhausted anyway!"
In this particular episode of why-am-I-not-exercising-good-self-care I find that the only way out is to soften to my own discouragement, to allow it to be there, to hold it tenderly and offer myself the kind of support that I would offer another. "Yes, I may wake up as tired as I am now. But I know, at some point this will change. There will be restorative rest again. Taking care of myself before bedtime is something I can change, even if the quality of sleep is something beyond my immediate control." And slowly, I coax myself into the self-care I know helps me enjoy the life I have been given.