Oxytocin (OT) is a hypothalamic peptide that enters the peripheral circulation via the posterior pituitary gland. OT plays a key role in regulating appetite, psychopathology, prosocial behavior and sexual function. Hypopituitarism is associated with increased obesity, increased psychopathology, sexual and prosocial dysfunction despite appropriate hormone replacement. A few studies suggest the existence of a possible OT deficient state in hypopituitarism. In animal models, melatonin has shown to increase OT release. This study is designed to evaluate oxytocin values after administration of melatonin in adults (healthy volunteers and patients with hypopituitarism). The investigators hypothesize that OT response will be blunted following melatonin in patients with hypopituitarism compared to healthy controls.