Authors report the findings of nocturnal polysomnography and multiple sleep latency test(MSLT) before and after morning light treatment in a winter depressive patient with hypersomnia. On polysomnographic recordings, the sleep pattern of this case before light treatment was similar to that of narcolepsy exhibited, sleep onset REM period(SOREMP). After treatment, the shortened REM latency changed to normal condition, but, deep sleep percentage did not changed, and stage 4 sleep percentage was decreased. Depressive symptoms were improved on clinical interview with Hamilton Depressive Rating Scale. Sleep log showed shortened sleep latency and reduced sleep duration. These findings suggest that although light treatment could alter the sleep structure in seasonal affective disorder with hypersomnia, it does not necessarily imply that antidepressant response of light treatment is result of change of sleep structure.