OBJECTIVE: To identify any relationship between abnormal interictal heart rate variability (HRV) during sleep and the occurrence of postictal generalized EEG suppression (PGES), a potential biomarker of sudden unexpected death in epilepsy (SUDEP), in patients with focal epilepsy. METHODS: We retrospectively reviewed 34 consecutive patients with focal epilepsy, who presented with focal to bilateral tonic-clonic seizures (FBTCSs), 19 with PGES (PGES+) and 15 without PGES (PGES-), and 14 patients without epilepsy as controls. HRV spectrum analysis was performed for periods of 10 minutes during wakefulness, non-REM sleep, and REM sleep. HRV spectra consisted of high frequency (HF) and low frequency (LF) regions. Normalized HF representing parasympathetic activities and normalized LF were compared between the PGES+, PGES-, and control groups for each sleep stage. RESULTS: The PGES+ group showed significantly decreased normalized HF and increased normalized LF during non-REM sleep compared to the PGES- (P < 0.05) and control (P < 0.01) groups, but not during wakefulness or REM sleep. CONCLUSIONS: Abnormal interictal HRV during non-REM sleep, indicating abnormally decreased parasympathetic activities, was associated with PGES in patients with FBTCS. SIGNIFICANCE: This study proposes to further investigate the relation between decreased parasympathetic activities and PGES.