As a dark and not fully understood side of human nature, yawning is believed to be a signs of various physiological or pathological behaviors of human. In this study, we aimed to investigate the importance of yawning in the evaluation of sleepiness. One hundred and twenty-nine snorers who were suspected to have obstructive sleep apnea syndrome underwent polysomnography and were asked to fill the Epworth sleepiness scale. The number of yawnings of patients was counted during the day following polysomnography. Patients were stratified into two groups: those have apnea hypopnea index < 5 (n = 43, group 1) and those have apnea hypopnea index > 30 (n = 86, group 2). Mean duration of sleep phases, oxygen saturations, sleep efficacies, yawning frequencies and Epworth scores of the groups were compared. Correlations of yawning frequency with Epworth scores, duration of sleep phases and mean oxygen saturations were investigated. Sleep efficacies were similar between the groups (p > 0.05). Yawning frequencies in group 1 and group 2 were 43.48 and 75.76 (mean rank), respectively (p < 0.01). Mean N1, N2, N3 phase durations and oxygen saturations were significantly lower in group 2 (p < 0.01). While there was a negative correlation between yawning frequency and duration of the non-REM phases and mean oxygen saturation (r = -0.53 and r = -0.31, respectively, p < 0.05), yawning frequency was positively correlated with Epworth scores (r = 0.46, p < 0.05). In addition to the shortened phases of sleep, increased Epworth score and decreased oxygen saturation, increased yawning frequency may indicate sleep deprivation.