Purpose:The differentiation of narcolepsy without cataplexy from idiopathic hypersomnia is based on the number of sleep-onset rapid eye movement periods (SOREMPs) observed by multiple sleep latency test (MSLT) and nocturnal polysomnography. The main aim of this study was to investigate the utility of SOREMP in differential diagnosis of central hypersomnias.Methods:The authors retrospectively evaluated consecutive 101 patients with a normal polysomnography other than the presence of SOREMP and/or REM without atonia and a latency of 8 minutes in MSLT.Results:The authors classified patients as follows: 52 patients had at least 2 SOREMPs (narcolepsy group), 23 had no SOREMPs (idiopathic hypersomnia group), and 26 patients had only 1 SOREMP (intermediate group). In polysomnographic recordings, both mean sleep latency and REM latency were significantly shorter in the narcolepsy (P = 0.012, P < 0.001, respectively) and intermediate groups (P = 0.005 and P = 0.035, respectively) compared with the idiopathic hypersomnia group. In MSLT recordings, sleep latency was 2.7 2.2 minutes in the narcolepsy group, 3.6 +/- 1.4 minutes in the intermediate group, and 5.2 +/- 2.7 minutes in the idiopathic hypersomnia group (P < 0.001). The mean REM latency and sleep stages SOREMPs arised from were similar between the narcolepsy and intermediate groups.Conclusions:To date, SOREMPs in MSLT and polysomnography remain the sole electrodiagnostic feature that discriminates narcolepsy without cataplexy from idiopathic hypersomnia. Different parameters or combined criteria are being increasingly investigated to increase the sensitivity and specificity of MSLT. The findings showed an altered instability of REM sleep not only in patients with 2 or more SOREMPs in MSLT but also in patients with one SOREMP.