When we think about optimizing our work capacity to produce desired outcomes, it is not solely a question of available time, but available energy, that matters. You may find that when you finally have some time to focus on an important project or task, you are too spent or burned out to actually do it.
Taking regular breaks is an essential part of managing our energy to maintain sufficient capacity — not only to be productive, but also to prevent burnout, as well as be able to maintain healthy self-regulation of our emotions and behaviors so we can have positive interactions with others. When we are fully depleted (or close to being depleted), we are more likely to be reactive, brusque, or irritable and take our frustrations out on others, which is not only unpleasant for those involved but can be counterproductive. Breaks are also particularly important when doing creative work. Read this great HBR article to find out more