SummaryThe present study is the first to prove the presence of hiccup during sleep polygraphically. Hiccup (Hc) penetrates all sleep stages; in REM sleep, it becomes randomized. The amplitude and frequency of sleep Hc have stage-dependent characteristics, and a linear regression appears with every sleep cycle. Sleep Hc may alternate, but does not coexist with, periodic leg movements. Related to inspiration, sleep Hc presents a right deviation when compared with wake Hc. The sleep pattern in persistent sleep Hc is disturbed in a nonspecific manner. Sleep Hc is not associated with sleep apnea. Sleep synchronizes the breathing rate with the hiccupping rate. During light sleep, the Hc rate exceeds the breathing rate, whereas during deep sleep, the breathing rate exceeds the Hc rate.