Figure 1 provides a representation of a 24-hour cycle of activity with current recommendations and a rough estimate of the proportion of SED to LPA. There has been a proliferation of wearable devices for the various components of daily activity, but little research into how these devices compare to one another and how valid and reliable they are compared to common research measurement methods. Technological advances now make these measurements possible, using small wearable devices. The collection of objective measures of sleep, SED, LPA, and MVPA has traditionally been costly, difficult, or non-existent. Accurate and reliable measurement of the 24-hour cycle could answer a number of these specific research questions that cannot be addressed with current measurement methods. The relationship among activity domains is also probably not stagnant, but changes across the life span, during specific physiological or disease states (i.e., pregnancy, diabetes), and with heavier physical training loads. For example, physical activity can be used as a treatment for poor sleep ( 6), but research has not addressed the need for more sleep (or sedentary time) as recovery following several days of extended vigorous intensity exercise. Importantly, the relationships among these activity domains are not well understood. There are no recommendations for how much of the day should be spent in LPA compared to sedentary behavior. Decreased LPA contributes to several health risks including elevated plasma glucose ( 15) and higher blood pressure and lower HDL cholesterol ( 8), but not mortality rates ( 24). Increased LPA is associated with improved physical health and well-being measures in older adults ( 5). Exercise is also related to multiple health outcomes ( 14), and this has led to public health recommendation of 150 min of MVPA per week to contribute substantially to longevity and disease prevention ( 14). Sedentary behavior recommendations are sparse ( 31), but objective monitoring of sedentary behavior has revealed relationships to a number of health outcomes ( 21), and several general recommendations have been published ( 13, 39). Sleep recommendations - to sleep for 7 to 8 hours per night – are based on observations that shorter or longer sleep durations are associated with risk factors for a range of diseases ( 7, 12, 34, 35). Lastly, there has ben a lack of analytical methods to consider time spent in different activity levels and the relative relationships to health outcomes. Second, measurement technology had both limited memory and short battery life. First, sleep, sedentary behavior and physical activity are traditionally studied in separate laboratories. A 24-hour model of activity was previously difficult to measure and incorporating the model into medical research was limited because of the error associated with the measurement. For all non-sleep activities, sedentary behavior is defined as sitting or lying with energy expenditure less than 1.5 METs ( 32), LPA would include activities with energy expenditure between 1.5 and 3 METs ( 1), and MVPA includes moderate activity (3–6 METs) and vigorous activity (any activity greater than 6 METs) ( 1). The components of the 24-hour model, organized into domains of activity intensity, are sleep, sedentary behaviors (SED), light-intensity physical activity (LPA), and moderate-to-vigorous physical activities (MVPA, or “exercise”). Current activity and sleep guidelines are limited to 30 minutes per day of exercise and 7–8 hours of sleep, leaving about 16 hours of unaccounted time with a non-quantified recommendation to avoid too much sitting. Combined measurement of sleep, sedentary behavior and physical activity may be an important step in guiding activity recommendations throughout a 24-hour cycle. Health research has focused intensely on these different daily activities, but in order for researchers, clinicians, and consumers to understand better these activity-health relationships, it is important to study the complete 24-hour activity cycle. Substantial evidence has led to recommendations for adequate exercise, healthy sleep habits, and limited sedentary behavior for increased longevity, improved health, and disease prevention ( 7, 14, 21).
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