Obstructive sleep apnea (OSA) is an age-associated disorder and its prevalence is increasing in Japan. OSA affects daytime physical and mental functions due to sleep fragmentation and hypoxia caused by apnea and hypopnea during sleep. OSA is associated with the development of depression and increases suicide scores in PTSD, but treatment by continuous positive airway pressure (CPAP) reduces the depressive symptom scores. OSA causes cognitive impairment across a range of domains and is linked to car accidents and dementia; however, CPAP reduces the risk of car accidents. Drowsiness, fatigue, and non-restfulness are some of the most frequent symptoms of OSA. There are two main methods to assess sleepiness: the assessment of subjective sleepiness, e. g. the Epworth Sleepiness Scale (ESS), and assessment of objective sleepiness, e. g. the Multiple Sleep Latency Test (MSLT) and the Maintenance Wakefulness Test (MWT). The ESS is a self-administered questionnaire designed to assess sleepiness in all aspects of daily life; however, the relationship between ESS scores and the apnea hypopnea index (AHI), which represents the severity of OSA, is controversial. The MSLT score is based on the mean latency to fall asleep at 5 daytime sessions, but its relationship with the AHI is unclear. Furthermore, although CPAP can improve ESS and MSLT scores, some patients still have excessive daytime sleepiness.
 According to previous studies that examined the relationship between sleepiness and EEG activity during sleep in OSA patients, the alpha waves and spindle waves are associated with subjective and objective sleepiness, respectively. Thus, it may be possible to estimate the degree of subjective or objective sleepiness in OSA by examining the physiological background of EEG activity during sleep rather than considering only the effects of subjective and objective sleepiness due to respiratory impairment variables of OSA.
 Authors' abstract