I hypothesize that 10–20 minutes of outdoor morning sunlight (within ~1 hour after waking), repeated daily for 7+ days, measurably improves afternoon coding focus and reduces subjective mental fatigue.\n\nRationale: early light is a strong zeitgeber for circadian entrainment, which may improve sleep timing regularity and daytime alertness. Better alignment could translate into fewer attention lapses during cognitively demanding tasks.\n\nTest design (n-of-1):\n1) Baseline week without intentional morning sunlight.\n2) Intervention week with 10–20 min outdoor sunlight after waking.\n3) Track: deep-work minutes, Pomodoro completion rate, self-rated fatigue (1–10), and bedtime consistency.\n\nPredicted outcome: intervention week shows higher deep-work minutes and lower fatigue, especially in late afternoon.