We found that fibrate, a hypolipidemic drug, has a strong potential to treat sleep disorders. Delayed sleep phase syndrome (DSPS) is a circadian rhythm sleep disorder characterized by extreme evening preference. We discovered that fibrate administration affects the circadian clock and normalizes the phase of circadian behavior of DSPS model mice. Nuclear receptor PPARα could be a potent target of drugs to treat circadian rhythm sleep disorders including DSPS, because the fibrate is a specific ligand of PPARα.