Energy management is a strategic approach to optimizing physical, mental, emotional, and social energy across daily work and life cycles rather than merely managing time. Unlike traditional time management, energy management recognizes that human performance fluctuates throughout the day in predictable patterns called ultradian rhythms (90-120 minute cycles). The core insight: productivity isn't about working longer hours—it's about aligning demanding tasks with peak energy windows and proactively renewing energy reserves before depletion occurs. Most professionals focus on filling their calendars, but high performers treat energy as their most valuable renewable resource, deliberately investing it where returns are highest.