Climbers of Mt. Fuji frequently suffer from altitude sickness. This happens if you climb all night without resting at a hut in the hope to see the sunrise from the summit, or if you plan a day trip and climb to high altitudes in one stretch. Lack of sleep can cause fatigue and even injury.

Altitude sickness, or Acute Mountain Sickness (AMS), is remarkably common on Mt. Fuji, affecting approximately 30% to 50% of climbers who attempt to reach the 12,388 ft (3,776 m) summit. Because many climbers start from the 5th Station (7,546 ft) and ascend rapidly within a single day—often "bullet climbing" overnight to see the sunrise—their bodies do not have sufficient time to acclimatize. Symptoms typically include headaches, nausea, dizziness, and extreme fatigue. Statistical studies show that first-time climbers and those who do not stay overnight at a mountain lodge are at the highest risk. In 2026, mountain authorities strongly advise the "Subashiri" or "Yoshida" paths with a managed pace, as the prevalence of AMS drops significantly for those who spend at least 6–8 hours at a 7th or 8th-station lodge. It is a peer-to-peer essential to carry supplemental oxygen canisters and descend immediately if symptoms become severe, as the rapid ascent remains the primary trigger.