The biggest challenge was philosophical, not technical. We initially assumed Flow was "good" and Drag was "bad" — but research, including Csikszentmihalyi's own warnings about flow addiction, completely flipped our framing. Designing for ADHD users we don't personally experience added another layer: everyone we interviewed had different symptoms at different strengths, so there was no single experience to design for. The biometric sensing model was our answer to balancing accuracy with the reality that ADHD users won't respond to constant manual prompts. The most important design lesson was restraint. No real-time state display (prevents obsessive checking), no scores or streaks (prevents shame spirals), evening reflection only. All data stays on-device, no social features, with a pause mode for grief or overwhelm. Designing for time blindness means designing for vulnerability — we took that seriously.