The major finding of this study was that the participants in the younger group with high fatigability scores had significantly lower %lag0 compared with those with low fatigability in the hour before sleep; however, the difference was not significant in the hour after waking up. Additionally, participants in the younger group with higher fatigability and exhaustion in the morning had significantly lower %lag0 than those without exhaustion in the hour before sleep but not in the hour after wake-up.
In younger participants, no significant differences were observed in the basic characteristics between the high and low fatigability groups. In the older group, however, the high fatigability group had significantly longer sleeping hours and lower PA in the evening than the low fatigability group. The sleeping hours at night also involved nocturnal awaking. A previous study reported that improved sleep may result in low fatigability in older people . Additionally, an experimental study has demonstrated that exercise has a positive effect on sleep in older adults with sleep compliments . Therefore, we believe that low PA before sleep is linked with low sleep quality and fatigability.
In the younger group, %lag0 one hour before sleep between HRV (both HFnu and LF/HF) and PA was significantly higher in the low fatigability group than that in the high fatigability group (Fig. 1). There were no such significant differences in the older group. It has been reported that fatigue stems not only from training overload or daily activities with inadequate rest but also from various inputs, such as psychological stress . Therefore, daily PA may be related to fatigability and, thus, affect HRV. Therefore, fatigability may be one of the factors that affects the coordination between HRV and PA in younger people.
Both, in younger and older participants, there were no significant differences in %lag0 one hour before sleep between HRV (both HFnu and LF/HF) and PA between the low and high fatigability groups. In the hour after waking up, the participants generally performed a higher number of daily activities than they did in the hour before sleep. Therefore, we believe that the hour before night sleep is the most appropriate time period to assess the relationship of fatigability and the coordination between HRV and PA. We also believe that %lag0 in the hour before sleep and that in the hour after waking up had different clinical meanings since %lag0 of these two time periods had no significant correlation (Appendix 1).
The participants who answered “yes” to the question “ Do you usually get up tired and exhausted in the morning?” had significantly lower %lag0 between HFnu and LF/HF and PA in the hour before sleep than those who answered “no”. This may indicate that discoordination between the autonomic nervous system and PA before sleep is related to fatigability after waking up. A previous report demonstrated that morning and evening fatigue may be distinct but related symptoms . Our findings could help distinguish the differences; we believe that fatigue in the morning is derived from the previous night. In this study, those who were employees had significantly higher fatigability than housewives, which may account for this difference. Further studies are required to elaborate on this topic.
In this study, we focused on evaluating the relationship between fatigability and HRV more clearly. A previous study demonstrated that mental fatigability and altered HRV are both related to selective regions of the prefrontal cortex, which build a functional circuit . In this study, younger women had significant differences in %lag0 of PA and HRV between the high and low fatigability groups in the hour before night sleep, although no significant differences were observed in HFnu or LF/HF before, during, or after night sleep. Therefore, we believe that fatigability may affect selective regions of the brain that, in turn, affect the coordination between HRV and PA rather than the autonomic nervous system. This index could be useful for public health or sports science in formulating measures to avoid fatigability.