The core problem with working longer hours is that time is a finite resource.
Energy is a different story. Defined in physics as the capacity to work, energy comes from four main wellsprings in human beings: the body, emotions, mind, and spirit.
In each, energy can be systematically expanded and regularly renewed by establishing specific rituals—behaviors that are intentionally practiced and precisely scheduled, with the goal of making them unconscious and automatic as quickly as possible.
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The full article can be read here on HBR: Manage your Energy, not your time.