Sleep-related breathing disorders are common causes of excessive daytime sleepiness, a socially and clinically relevant problem. Mechanisms responsible for daytime sleepiness are still largely unknown. We investigated whether specific alterations in autonomic cardiac modulation during sleep, commonly associated with sleep-related breathing disorders, are related to excessive daytime sleepiness. Fifty-three patients with sleep-related breathing disorders underwent nocturnal polysomnography. Excessive daytime sleepiness was diagnosed as a Multiple Sleep Latency Test response less than or equal to 600 s. We explored the relation of excessive daytime sleepiness, objectively determined, with indices of autonomic cardiac regulation, such as baroreflex sensitivity and heart rate variability, with polysomnographic indices of the severity of sleep-related breathing disorders and with quality of sleep. Patients with excessive daytime sleepiness, when compared with patients without, had significantly lower baroreflex sensitivity and significantly higher low-to-high frequency power ratio of heart rate variability during the different stages of nocturnal sleep. By contrast, no differences were found in indices quantifying the severity of sleep-related breathing disorders or sleep quality. We demonstrated that excessive daytime sleepiness is accompanied by a deranged cardiac autonomic control at night, the latter probably reflecting autonomic arousals not detectable in the EEG. As abnormal autonomic regulation is also known to be associated with increased cardiovascular risk, a possible relation between excessive daytime sleepiness and cardiovascular events in patients with sleep-related breathing disorders deserves to be investigated in future studies.