The head of the Mental Health Institute at Kangbuk Samsung Medical Center surveyed 202,629 workers in their 20s, 30s and 40s who had visited the hospital for a health checkup in 2014. The study showed that among people who sleep four hours or less a day, the prevalence of depression was 9.1 percent, compared to 2 percent among people sleeping seven hours, the base condition set for the survey.

Prevalence of anxiety and suicidal thoughts for the same group were 16 percent and 12.7 percent, respectively, compared to 4.3 percent and 5 percent for people who sleep the standard seven hours, the study said.

Signs of mental harm, although to a lesser degree, also showed among people who sleep more than seven hours, according to the study.