I would suspect the physiological base of burnout is the depletion of mainly B vitamins in the brain. When you look at the Krebs cycle, the main energetic reaction in every cell, B vitamins play crucial role whether they act as catalysts of enzymes (B1, B2), provide electron transport (B2/B3 to FAD/NAD+) or are required by sub-processes of the cycle (B5, B7, B12). Being under constant stress is known to deplete B vitamins and at some point their lowered availability starts inhibiting higher cognitive tasks in favor of just survival. Diagnosing this is problematic as serum levels don't tell much about tissue levels.