Impact of Age at Narcolepsy Onset on Sleep-Onset REM Periods in the Multiple Sleep Latency Test

<b>Background/Objectives:</b> This study aimed to investigate the effect of age at symptom onset on rapid eye movement (REM) sleep latency and sleep-onset REM period (SOREMP) distribution in multiple sleep latency tests (MSLTs) in patients with narcolepsy. <b>Methods</b>: This was a retrospective multicenter chart review of 135 newly diagnosed drug-naïve patients with narcolepsy who underwent MSLT and fulfilled the diagnostic criteria for narcolepsy. The age at onset was defined as the first occurrence of excessive daytime sleepiness or cataplexy. We investigated sleep onset latency, REM sleep latency, and the presence of SORMEP in each nap trial of the MSLT. The clinical, polysomnography, and MSLT findings were compared between the early- and late-onset groups. Correlation and linear regression analyses were used to assess the effect of age at onset as a continuous variable, and survival analyses confirmed its impact on the MSLT parameters. <b>Results</b>: The mean age at onset was 18.3 ± 8.8 years. Patients with early onset had a higher rate of SOREMPs than late-onset patients in the first MSLT nap (81.9% vs. 63.3%, <i>p</i> = 0.031). However, the severity of the narcolepsy symptoms did not differ between the groups. In linear regression analysis, age at onset was significantly associated with MSLT REM sleep latency (β = 0.049, <i>p</i> = 0.033) after adjusting for confounders. Survival analysis confirmed that an early onset of narcolepsy was associated with a higher probability of SOREMPs in the first MSLT nap (hazard ratio 0.955, <i>p</i> = 0.001). <b>Conclusions</b>: A younger age at narcolepsy onset was associated with shorter REM sleep latency and higher SOREMP probability in MSLT. These findings indicate that the early onset of narcolepsy may be linked to greater disease severity in terms of REM sleep dysregulation.