Forty participants (20 males and 20 females; age, 17–36 years; mean age, 25.7 ± 7.1 years) were recruited for the sleep-position study. The participants claimed that they were healthy, had no sleep disorders, and did not have important work to do the next day, so they were able to sleep without stress. In this way, all sleep positions could be recorded for further analysis. All participants were voluntary subjects and were paid the equivalent of 35 USD for participation.