In adult women, we confirmed that %Lag0 is reduced with aging. It was significantly lower in older subjects before and after sleep than in younger subjects, as reported in our previous study . Furthermore, older women with depression had significantly lower %Lag0 in the hour before sleep. These results indicate that depression is closely associated with impairments in the coordination between HRV and PA before sleep in older women.
Psychological stress can be accurately evaluated using the total score of GHQ28 . These findings appear to support our present results in older and younger individuals with depressive tendencies based on their GHQ28 subscale D scores and lower %Lag0 between HRV and PA; however, the decrease was not significant in younger individuals. These results indicate that depression is closely associated with discoordination between HRV and PA only in older women, particularly in the hour before sleep. To the best of our knowledge, this study is the first to focus on the association of depression with coordination between HRV and PA in women.
By analysing %Lag0, this study suggested that mental health conditions are closely associated with the coordination between HRV and PA in older women. The hypothalamic pituitary adrenal axis plays a significant role in controlling neuroendocrine stress responses . In vivo human imaging studies reported that hypothalamic pituitary adrenal axis responses to a number of different stressful stimuli are gated through various brain regions [30, 31]. All activities in the body that are initiated from the telencephalon, including those controlled by neuronal programs in the hypothalamus and mesencephalon, co-opt the lower brain stem centres . PA is considered to be linked with the autonomic nervous system . Therefore, impairments in the coordination between the HRV and PA may be derived from compromised links among their systems.
Patients with depression have been reported to have sleep disorders . Participants with sleep difficulties are threefold to fourfold more likely to be depressed, and those with depression have low sleep efficiency . In this study, no significant difference in %Lag0 was detected between participants with or without incidents of nocturnal awakening on the experimental day or irregular sleep cycles, based on the questionnaires (data for reviewers). In the groups with depression, a close relationship was observed between HRV and depression before and during sleep, reflecting the coordination between HRV and PA.
The menopausal years in Japanese women reportedly begin at approximately 52 years of age. We divided subjects into 2 groups, namely, the younger group (59 years or younger) and the older group (60 years or older). Therefore, the older group might comprise only postmenopausal women. In contrast, some of the subjects in the younger group might also have been postmenopausal, as they were in their late forties and early fifties. Estrogen has been reported to reduce cardiomyocyte contractile function and sympathovagal nervous activity, whereas androgen enhances these . Estrogen acts on the central nervous system to reduce sympathovagal activity . In postmenopausal women, sympathovagal nervous activity is elevated due to changes in the hormonal balance . Menopause or the menstruation cycle could also affect depression or distress .
Depression is strongly associated with physical disability , moreover, the prevalence of depression in patients with cardiac disease is quite variable . We consider that our new index could help understand the connection between physical function and depression, and thus reduce cardiovascular diseases.
The analytical method used in this study to evaluate the coordination between the HRV and PA has several limitations. We defined the beginning and end time points of a night’s sleep based on changes in body position and the records of participants. Therefore, accurate identification of falling asleep, nocturnal waking, and waking up was not possible. In addition, as discussed above, we analysed the coordination between HRV and PA in the hour before sleep. We initially considered the higher sensitivity of %Lag0 in the hour before sleep to be derived from equal activity patterns with fewer variations in PA. However, no significant differences were observed among PA, average PA, or coefficients of variation during the waking periods, even in the hour before and after sleep (data for reviewers). Physiological reasons for why the hour before sleep is the key time to examine the relationship between %Lag0 and psychological distress remain unclear. In the present study, we could not establish whether menopause affected the %Lag0 in younger and older women.
This study did not control for the comorbidity score or physical condition of participants. All participants had no or mild diseases. However, we considered that various kinds of metabolic and/or neuromuscular diseases could be involved in the discoordination. As the number of participants was limited, the effects of diseases on %Lag0 remained unclear. Taking the above limitations into consideration, further controlled studies are required in the future.