# Influence of Posture and Sleep Duration on Heart Rate Variability in Older Subjects
## 1 Introduction  
Japan is facing an aging society, and with the decline in the birthrate and the resulting decrease in the number of young workers, the country is encountering challenges such as an increased burden on social security systems and a rise in the number of elderly workers. One measure to address this situation is to extend the healthy life expectancy, which necessitates lifestyle improvements, such as frailty prevention and ensuring adequate sleep.  
However, as we age, our basal metabolic rate decreases. This decline results in nutrients consumed not being efficiently used as energy but instead stored as fat in the body. Additionally, as energy is not properly distributed throughout the body, the functions of blood vessels, internal organs, and muscles deteriorate. A reduction in lean body mass, such as muscle, further contributes to decreased energy metabolism during activity. While the primary cause of the decline in basal metabolic rate associated with aging isattributed to the loss of skeletal muscle mass, this factor alone does not fully account for the phenomenon. A decrease in the metabolic rate of individual organs is also considered a significant contributor. Numerous studies support this view, including those related to elderly metabolism [1; 2; 3], elderly pharmacokinetics [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15], and energy requirements and aging [16; 17; 18; 19; 20], which highlight unique metabolic phenomena in the elderly.  
In recent years, new findings have emerged regarding the activities and sleep of the elderly. For example, exercise has been emphasized as crucial for preventing obesity, which occurs when energy intake exceeds energy expenditure [21]. Additionally, a short sleep duration has been identified as a risk factor for obesity [22].  
Heart rate variability (HRV), widely recognized as an indicator of autonomic nervous system function, has become an essential tool for assessing the health of the elderly. Studies have shown a correlation between heart rate indicators and the basal metabolic rate, indicating that a higher basal metabolic rate is associated with a higher pulse rate. HRV is also used as a measure of the body's ability to respond to physiological stress and maintain overall health [23; 24; 25; 26; 27; 28; 29]. A decrease in HRV is mainly related to a decrease in basal metabolic rate and is an indicator of the amount of energy the body consumes when at rest, and it tends to decrease with age and changes in health. When the basal metabolic rate decreases, the body's energy consumption decreases, and the body's supply of energy needed for recovery and maintenance also decreases. This also affects the need for energy during sleep, and it has been found that the restorative effects gained during sleep decrease. The body tries to maintain daily activities with less rest, and this is one of the factors that causes a decrease in HRV. HRV is an indicator of the balance of the autonomic nervous system, and because the mechanisms of energy consumption and recovery are affected, a decrease in basal metabolism also has a direct effect on HRV [30; 31; 32; 33].  
Therefore, in this study, we focused on the relationship between heart rate variability and sleep duration, aiming to quantitatively evaluate this relationship.