There was a study by Kripke showing that many sleeping pills, namely benzodiazepines, tripled your likelihood of death even if taken very infrequently. Zopiclo

There was a study by Kripke showing that many sleeping pills, namely benzodiazepines, tripled your likelihood of death even if taken very infrequently. Zopiclone is not a benzodiazepine but I don't know if it has been studied in a similar manner.