Depression may be related to a condition called sleep apnea, in which a person stops breathing from time to time in his or her sleep. A study by the Centers for Disease Control and Prevention found that people with this condition are more prone to show depressive symptoms such as hopelessness and disinterest.
According to the study, men diagnosed with sleep apnea were twice as likely to show depressive symptoms as men without; among women, the risk was five times as much. Furthermore, the researchers found that four in five people reporting apnea symptom—including loud snoring, gasping for breath on waking, morning headaches—have never been diagnosed.
The link between apnea and depression was found even after correcting for various apnea risk factors, such as being overweight. Overweight people without apnea did not show depressive symptoms as much as people with apnea.
Researchers warn that the study only shows that apnea and depression are associated, with no indication whether either causes the other, and if so, which is the cause. However, it’s highly likely that the sleep deprivation caused by sleep apnea causes depressive symptoms such as lethargy.