In order to test whether exposure to bright artificial light at night is a necessary condition for the antidepressant response to sleep deprivation therapy, five patients were totally sleep-deprived on two separate nights, once in very bright light and once in nearly total darkness. During the day after the sleep-deprivation night patients were found to have responded equally well to sleep deprivation in both conditions. During the sleep-deprivation night, however, antidepressant responses may have been greater in the bright light condition. Thus, light at night is not necessary for the antidepressant response to sleep deprivation, but we cannot rule out the possibility that the effects of light exposure and sleep deprivation are additive or that exposure to light at some time after sleep deprivation begins (including during the following day) is necessary for the response.