Sleepiness and sleep efficiency in patients with OSA. Subjective perception of sleepiness defined by ESS starts to decrease in older people with OSA, as the AHI and respiratory related arousals de-crease, despite the sleep latency and WASO periods are prolonged and deep sleep efficiency is below 80%. ESS - Epworth sleepiness score pre-polysomnography, WASO - wake after sleep onset, /hTST - events per hour of total sleep time.