Similarly, a study investigating the relationship between sleep time and psychomotor vigilance performance reported a sleep need of 8.16 h for preventing cumulative neurobehavioral deficits 18. However, little is known about the amount of sleep need to maintain normal functioning of the mind and body.Įpidemiological studies have expressed the relationship between sleep time and various health risks (e.g., cardiovascular disease 8, 9, 10, obesity and metabolism 11, 12, depression 13, and mortality rate 14, 15, 16, 17) as a U-shaped curve with 7–8 h at the trough. Sleep loss (or sleep debt) causes sleepiness and a decline in performance 2, 3 and also impairs many other psychological and physical functions such as memory, learning 4, metabolism 5, and immunity 6, 7. The US Centers for Disease Control and Prevention reports that 30% of workers in the United States sleep less than 6 h/day 1. Sleep is an essential behavior that takes up around one third of a human lifetime, but securing sufficient sleep time is difficult in modern society. To establish appropriate sleep habits, it is necessary to evaluate OSD, vulnerability to sleep loss, and sleep homeostasis characteristics on an individual basis. These findings confirmed self-evaluating the degree of sleep debt at home as a useful clinical marker. Additionally, the increase (rebound) in total sleep time from HSD at the first extended sleep would be a simple indicator of PSD. Recovery from PSD was also associated with the improvement in glycometabolism, thyrotropic activity and hypothalamic-pituitary-adrenocortical axis. Furthermore, only 1 h of PSD takes four days to recover to their optimal level. We found that OSD varies among individuals and PSD showed stronger correlation with subjective/objective sleepiness than actual sleep time, interacting with individual’s vulnerability of sleep loss. In this study, we hypothesized that dynamics of sleep time obtained over consecutive days of extended sleep in a laboratory reflect an individual’s optimal sleep duration (OSD) and that the difference between OSD and habitual sleep duration (HSD) at home represents potential sleep debt (PSD).
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