# Influence of Posture and Sleep Duration on Heart Rate Variability in Older Subjects
## 4 Discussion  
From these results, it can be surmised that the group with a short sleep duration (G4) was leading an active lifestyle and that HRV was declining, based on the decrease in MRRI and SDRR and the increase in LF/HF. In addition, the group with a long sleep duration (G1) showed a decrease in SDRR and ULF despite an increase in MRRI and a decrease in LF/HF. This suggests that HRV declines with age, and that this effect is particularly pronounced in older people. The findings of this study indicate that HRV indices decrease with aging, which is largely consistent with previous studies. On the other hand, we succeeded in uncovering a novel insight that has not been thoroughly explored in prior research--namely, that groups with shorter sleep durations tend to exhibit higher levels of physical activity. This result provides an important contribution to understanding the relationship between sleep and physical activity. Numerous studies have investigated the relationship between sleep duration and lifespan [36; 37; 38; 39; 40; 41; 42], showing that sleep duration shortens with aging, sleep becomes more fragmented, and its quality declines. For example, children up to adolescence typically require about 9 h of sleep, which decreases during the mid-teenage years, stabilizing around 7 h in the twenties. In older adults, sleep duration is suggested to decrease further, to about 5-6 h [43]. This reduction is partially attributed to a decline in basal metabolic rate, leading to a lower energy replenishment need through sleep. Heart rate variability (HRV) generally tends to decline with age. Specifically, as people age, the balance between the sympathetic and parasympathetic nervous systems changes, and in particular, the activity of the parasympathetic nervous system (which works when we are relaxed) decreases, so HRV tends to decrease [44; 45; 46; 47; 48; 49; 50]. With age, indices such as the standard deviation of heart rate variability power (SDRR) and RMSSD decrease. These indices mainly reflect parasympathetic nervous system activity, and as the parasympathetic nervous system becomes less active with age, these indices also decrease. With age, the LF/HF ratio, which is the ratio of low-frequency and high-frequency components, may increase, but this is due to the relative increase in sympathetic nervous system activity. Changes in heart rate variability indices over time are generally observed as physiological changes. In terms of gender differences, it has been reported that women have significantly lower mean RR intervals and standard deviations of RR intervals (SDNN), higher high-frequency (HF) values and lower low-frequency (LF) values in the power spectrum density of HRV, and lower total power [51; 52; 53]. Among older adults, going to  
Figure 3: HRV indicators for each group: (**a**–**c) indicate the HRV indexes for 2019, 2020, and 2021, respectively. The symbols in the figure are as follows, * \(p\) < 0.05, **_p_ < 0.01 vs. G1, + \(p\) < 0.05, ++ \(p\) < 0.01 vs. G2, # \(p\) < 0.05, ## \(p\) < 0.01 vs. G3. bed early despite a lack of sleepiness may contribute to a poor sleep onset and increased instances of waking up during the night. From this perspective, rather than focusing on sleep duration, increasing physical activity may be a more desirable strategy for maintaining health in older age. However, it has also been reported that sleep quality declines as the number of chronic conditions increases with age [37, 38, 39, 40, 41, 42, 43, 54]. This suggests that unresolved issues remain regarding the relationship between HRV and sleep quality. And there are several potential limitations to the analysis using the ALLSTAR (Allostatic State Mapping by the Ambulatory ECG Repository) database. Firstly, when analyzing the combination of electrocardiogram (ECG) and accelerometer data, there are individual differences in Holter electrocardiographs, which are corrected for, but there is a possibility that differences in data consistency and accuracy will arise.