Similar roles have also been associated with sleep; in fact, sleep, circadian rhythm and energy balance (hence, body weight) have been suggested to have parallel relationships via their intrinsic link with nutrient oxidation, ghrelin and leptin levels, appetite, food-reward homeostasis, hypothalamic-pituitary-adrenal (HPA) axis, with some of these also being critical factors in depression [94]. The gene discussed is LEP; the disease is major depressive disorder.