Sixty-one papers involving over 1,700 subjects have documented that over half of depressed patients experience an antidepressant response to sleep deprivation. Eighty-three percent of unmedicated depressed patients who had an antidepressant response to sleep deprivation relapsed after one night of sleep. Short naps can also activate severe relapses. The authors suggest that these phenomenological observations concerning relapse with a night of sleep or with naps after successful sleep deprivation would be compatible with the existence of a sleep-associated depressogenic process.