The Link Between Various Sleep Disorders and the Onset of Depression: What Findings Did the Population Study in Denmark Bring?

Introduction

The quality of sleep and the circadian sleep/wake rhythm are important building blocks for human mental health. At the same time, sleep disorders are an integral part of virtually all mental illnesses. Insomnia and hypersomnia are therefore, for example, essential criteria in the diagnosis of mood disorders. Many studies confirm an increased incidence of depression in patients with sleep disorders: insomnia, hypersomnia, obstructive sleep apnea, narcolepsy, or restless legs syndrome.

Evaluated Population

In this case-control study, longitudinal data from Danish population registers, which include all citizens of the Kingdom of Denmark, were used. Patients who were first diagnosed with depression in Denmark between 1995–2013 were evaluated (a total of 65,739 individuals). For each case, 20 healthy controls of the same sex, year and month of birth were randomly selected (a total of 1,307,580 individuals).

Depression prevails in the female gender not only in the Danish population, which was reflected in this study with a 62.7% representation of women. Socioeconomic parameters were also evaluated.

Findings

As expected, a higher risk of depression was found in people with lower income, shorter education duration, unemployment, single status, and those living in large cities. An increased risk of developing depression was proven in all evaluated sleep disorders (insomnia, hypersomnia, narcolepsy, circadian rhythm sleep/wake disorders, parasomnias, abnormal movements during sleep, narcolepsy, ventilatory dysfunctions during sleep including obstructive sleep apnea syndrome).

Overall, the risk of developing depression in patients with a sleep disorder is approximately 2.33 times higher compared to the general population. The highest risk concerns patients with circadian rhythm sleep/wake disorders and nonorganic insomnia, while the lowest concerns people suffering from narcolepsy. The higher risk of developing depression was also noted in individuals diagnosed with a sleep disorder in the past six months, compared to those who had a sleep disorder for at least one year.

Discussion

The relationship between sleep disorders and depression is significant. There is increasing evidence pointing to the possibility of preventing some psychiatric disorders through consistent treatment of sleep disorders. If sleep disorders are addressed early and thoroughly, it can potentially prevent the onset of depression.

The most vulnerable group is patients who have recently developed a sleep disorder. However, those who have been dealing with a sleep disorder for a long time remain at risk, especially when therapy is not effective. Therefore, patients with sleep disorders should be given increased attention in terms of screening and diagnosing depression.

Summary and Conclusion

Sleep is an important and inseparable part of a healthy life. Sleep disorders contribute to all mental disorders and can play a significant role in their etiopathogenesis. Based on extensive data from Danish population registers, the study authors concluded that sleep disorders – particularly circadian rhythm sleep/wake disorders and non-organic insomnia – represent a significant and non-negligible risk factor for the onset of depression. From this perspective, narcolepsy and ventilatory dysfunctions during sleep, except for obstructive sleep apnea, are the least risky.

Another interesting piece of information is that with the increasing duration of a sleep disorder, the risk of developing depression decreases. However, even after years of a sleep disorder, the risk of depression remains higher compared to the general population. For clinical practice, it is thus worth considering screening for depression in patients with sleep disorders, even with longer duration.

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Source: Byrne E. M., Timmerman A., Wray N. R., Agerbo E. Sleep disorders and the risk of incident depression: a population case-control study. Twin Res Hum Genet 2019 Jun; 22 (3): 140–146, doi: 10.1017/thg.2019.22.