The invention provides agents for phase-adjusting or enhancing the amplitude of an endogenous melatonin secretion rhythm and for improving a circadian rhythm, which may be taken daily and continuously, have excellent safety, and may effectively prevent or ameliorate disorders of an endogenous melatonin secretion rhythm or of a circadian rhythm without administration of exogenous melatonin, as well as functional food containing such an agent, which may prevent or ameliorate various symptoms, such as sleep disorder or prolonged sleep latency. The agent for phase-adjusting or enhancing the amplitude of an endogenous melatonin secretion rhythm according to the invention contains whey as the active component. The functional food for improving an endogenous melatonin secretion rhythm according to the invention contains the agent for phase-adjusting or enhancing the amplitude of an endogenous melatonin secretion rhythm. The functional food for improving a circadian rhythm contains the agent for improving a circadian rhythm.
