The digital era has magnified human attention scarcity, turning it into a pivotal resource for well-being and productivity. This paper introduces the Attention Grains (AGs) Model and its Nurture-Drain-Invest (NDI) metabolism as a novel framework to understand, quantify, and manage the brain's "cognitive fuel". Each AG constitutes the smallest functional unit of attentional effort, while the NDI metabolism explains how these units are replenished, depleted, and strategically allocated over time. We provide (i) interdisciplinary theoretical grounding, (ii) a formal AG balance equation, (iii) an experimental blueprint for behavioural and neurobiological validation, and (iv) an open research agenda. The model proposes testable metrics for cost (β) and basal capacity (AG₀), opening new avenues for ethical platform design, digital-well-being interventions, and human-AI alignment.