Hormonal

Shifting the sleep/wake schedule relative to the endogenous circadian rhythm (as seen in shift work) increases the daily peak-to-trough fluctuation of circulating leptin, resulting in lower leptin levels upon awakening.

If you work shifts or have an irregular sleep schedule, your body's internal clock may be out of sync with your sleep times, causing your leptin (satiety hormone) to drop significantly when you wake up. This low leptin state can increase appetite and food intake. To mitigate this, try to align your sleep schedule as closely as possible with your natural circadian rhythm, or use bright light exposure upon waking to help reset your internal clock.