I study how the body’s internal clock—known as the circadian system—affects mental health, with a focus on depression. Although disruptions in circadian rhythms have been linked to depression, it remains unclear which alterations are most relevant and whether they play a causal role. I use a combination of methods, including self-report questionnaires, actigraphy (to track activity), and biological markers such as melatonin, cortisol, and body temperature.