Habits, light, genes
Delayed sleep phase
syndrome is a partly genetic condition whereby one's sleep-wake schedule is greatly delayed with respect to the timing of one's internal biological clock.
Causes include 1) habitually intentionally staying up late (e.g. To play video games), 2) exposure to too much light before bedtime, 3) exposure to too little light upon awakening, 4) genetic predisposition, 5) certain meds.