Study objectives: Sleep-onset rapid eye movement (SOREM) during daytime naps is recognized as a main diagnostic feature of narcolepsy. However, SOREMs have been reported to occur in other disorders. This study set out to answer three questions: 1) whether the majority of patients with SOREMs are diagnosed with narcolepsy; 2) if the number of SOREMs is linked with the degree of daytime sleepiness; and, 3) whether patients with SOREMs are sleepier than patients without SOREMs. Methods: One hundred and eighty-five charts of sleep clinic patients with SOREMs on the Multiple Sleep Latency Test (MSLT) or Maintenance of Wakefulness Test (MWT) were compared to 178 charts from clinic patients without SOREMs on the MSLT or MWT (control group). Information was collected from the initial, diagnostic sleep study. Results: Patients with SOREMs were almost as frequently diagnosed with narcolepsy as with obstructive sleep apnea (OSA) or depression/anxiety. Subjective measures of sleepiness, alertness and fatigue were not different between the SOREM and control groups. The SOREM group did not exhibit shorter mean sleep onset latencies on the MSLT or MWT but a greater number of SOREMs was associated with increased sleepiness on the MSLT, but not on the MWT or subjective measures of sleepiness. Conclusion: SOREMs occur across a wide variety of sleep and psychiatric disorders. Patients with SOREMs were not sleepier, more fatigued or less alert than those without SOREMs. The findings of this study indicate that SOREMs are not an accurate or specific diagnostic marker of narcolepsy.