Objective To explore the relationship between hemodynamic changes during deep sleep and cognitive impairment in patients with epilepsy.Methods One hundred and two patients with epilepsy who were admitted to the hospital from February 2015 to February 2020 were enrolled in epilepsy group， which was further divided into cognitive impairment group ［n=35， Montreal Cognitive Assessment （MoCA） score<26］ and non-cognitive impairment group （n=67， MoCA score ≥26）； 32 healthy subjects who underwent physical examinations in the hospital were enrolled in normal control group. The brain waves and hemodynamic changes during deep sleep （N3+N4 stage） of all included subjects were monitored simultaneously， and their relationships with cognitive function were analyzed.Results The cognitive impairment group had significantly lower age of onset and significantly increased duration of disease， number of monthly seizures， and proportion of generalized seizures compared with the non-cognitive impairment group （P<0.05）. Compared with the normal control group， the cognitive impairment group and non-cognitive impairment group had significantly longer rapid eye movement （REM）， N1， and N2 stages and a significantly shorter N3+N4 stage （P<0.05）. The cognitive impairment group had significantly longer REM and N1 stages but significantly shorter N3+N4 stage and total sleep time compared with then on-cognitive impairment group（P<0.05）.The cognitive impairment group had a significantly lower interictal epileptiform discharge index during deep sleep and a significantly higher spike-wave index during non-REM sleep compared with the non-cognitive impairment group （P<0.05）. Compared with the non-cognitive impairment group and normal control group， the cognitive impairment group had significantly increased mean cerebral blood flow velocity （CBFVm） of the middle cerebral artery and degree of CBFV fluctuation during deep sleep （P<0.05）；there were no significant differences in CBFVm and degree of CBFV fluctuation during deep sleep between the non-cognitive impairment group and the normal control group （P>0.05）.Conclusions There are obvious hemodynamic changes during deep sleep in patients with epilepsy， and the CBFV fluctuation is associated with the decreased cognitive function of the patients.