Summary We assessed daytime sleepiness using objective and subjective measures in women with severe premenstrual syndrome (PMS) compared with women without significant premenstrual symptoms. Nine women with severe PMS and eight controls (aged 18−40 years) completed a laboratory-based daytime protocol including the maintenance of wakefulness test (MWT), psychomotor vigilance task (PVT), quantitative waking electroencephalogram (EEG), auditory and visual event-related potentials (ERPs), and sleepiness and mood scales during the mid-follicular and late-luteal (premenstrual) phases of the menstrual cycle. In association with increased perceived sleepiness, fatigue and other premenstrual symptoms in the late-luteal phase, women with PMS performed more poorly on the PVT, with increased lapses and slower reaction times (P <0.05), compared with the follicular phase and controls. However, there were no significant group or menstrual phase differences in latency to sleep on the MWT. Waking spectral EEG power and ERP measures also did not differentiate PMS women when symptomatic. Both groups of women displayed increased spectral power in the delta/theta frequencies (2−6 Hz) and fast alpha frequency (11−12 Hz) in the late-luteal phase relative to the follicular phase. Trait-like differences were apparent in that women with PMS had increased beta1 (12−16 Hz) power and smaller P300 amplitude than controls in both menstrual cycle phases. Our findings indicate that women with severe PMS are subjectively sleepy and fatigued, and show psychomotor slowing when symptomatic compared with when they are symptom-free and compared with controls. However, the ability to maintain wakefulness under soporific conditions, spectral properties of waking EEG and cognitive processing do not vary in synchrony with premenstrual symptoms.