Introduction
Infertility is a reproductive system disease that is characterized by the inability to achieve a clinical pregnancy after 12 or more months of regular unprotected sexual intercourse 1. According to epidemiological surveys, the incidence of infertility is rising and trending younger, affecting 186 million couples worldwide2, accounting for about 10%-15% 3, with a prevalence of 25% among couples of reproductive age in China4, which has become the third most serious disease affecting human health after cardiovascular diseases and tumors5. With the implementation of China’s fertility policy, infertile individuals and couples are under pressure from society, family, and the treatment process due to the long-term inability to have children normally, causing great psychological burden and social-emotional distress, which largely affects the physical and mental health of infertility patients 6, 7.
Depression, one of the most prevalent mental disorders, is manifested by persistent low mood, affects 4.4% of the global population proportionally, or approximately 322 million people 8, and is the second leading cause of disability worldwide9. Currently, it is well documented that depression is closely associated with an increased risk of developing multiple metabolic syndromes 10, including obesity11, hypertension 12, and immuno-metabolic dysregulation 13, resulting in long-term adverse health outcomes. However, regarding the comorbidity phenomenon of depression and infertility, it is unclear whether this simply reflects shared risk factors. There is speculation that depression may be a cause of infertility, its consequence, or both14. A previous study reported a significant association between depression and treatment outcomes in assisted reproductive technology (ART) 15. As a matter of fact, depression may be even more prevalent in infertile patients compared to the general population of reproductive age 14, considering the long course of infertility, complex etiology, many cooperating factors, high treatment costs, cumbersome treatment process, and the influence of social culture and traditional beliefs. There is growing evidence that depression may be associated with multiple factors, not only risk factors from social aspects but also individual-level lifestyle factors, such as circadian rhythm disturbances, which may play a significant role in the onset and progression of depression and the overall severity of depressive symptoms 16.
Accumulating evidence suggests that circadian rhythm disturbances are associated with a number of adverse health outcomes17, including but not limited to obesity18, type 2 diabetes 19, cardiovascular diseases 20, reproductive functions21, psychiatric disorders 22, and cancer 23. The chronotype is generally considered to be one of the manifestations of circadian rhythms, and it represents the behavioral pattern of organizing events during the 24 hours of a day, especially referring to the subjective preference of individuals for sleep-wake times 24, 25, which can be divided into morning chronotype, neutral chronotype, and evening chronotype26. In previous studies, it was agreed that evening chronotype was a risk factor for mental health and that individuals with morning chronotype were at lower risk of developing emotional problems such as anxiety and depression compared to evening chronotype27. With the increasing prevalence of infertility, more and more researchers are also focusing on circadian rhythm variations in infertile individuals and have found that worse sleep quality and evening chronotype are more common in infertile populations compared to those with fertility 28. However, it is noteworthy that two prospective studies exploring the onset and progression of infertility from a chronotype perspective suggest suggested that morning chronotype may be a risk factor for fertility29, 30.
To be sure, it is necessary to examine the impact of morning chronotype on depression during infertility treatment. Since infertility is a more specific reproductive disorder, although not fatal, it can have a negative impact on the individual and the spouse, or even the entire family. Depressive symptoms, a major type of depression, without timely intervention are likely to develop into depression. Therefore, exploring the causes of depression is essential to promote health and function throughout the life course. To the best of our knowledge, there is a lack of clarity about the relationships between chronotype and depressive symptoms during infertility treatment. Accordingly, the objectives of this study were (1) to examine the effect of sleep type on the depressive symptoms in subjects undergoing infertility treatment. and (2) to assess the partner effect of spouses’ chronotype on each other’s depressive symptoms.