Slow waves during sleep are fundamental for neural homeostasis, metabolic regulation, and waste clearance, and are known to be altered in neurodegenerative diseases. Sleep-like slow waves (SLSW) have been observed in awake healthy individuals, where they are linked to fluctuating vigilance and mind-wandering. Whether SLSW are altered in Parkinson’s disease (PD), and whether such alterations contribute to psychiatric or cognitive complications, remains unknown. In a cohort of 84 non-demented PD patients and 30 healthy older adults, we found that SLSW occurred more frequently in PD. Hallucinatory traits in PD were associated with higher SLSW amplitude, especially over fronto-central regions, and the extent of SLSW alterations was associated with the severity of hallucinatory burden. Moreover, more prominent SLSW predicted greater cognitive impairment. We conclude that, while slow waves during sleep have positive physiological functions, exacerbated diurnal SLSW may represent reactions related to the PD neuropathology, at the cost of hallucinations and cognitive impairment.