role in hippocampal-dependent memory. However, recent studies have shown that several factors can interrupt neurogenesis, including stress and prolonged sleep deprivation (more than one day). The sleep disturbances encountered in AD could therefore suppress neurogenesis—and thus impair hippocampal functions. This would contribute to diminished memory performances and the progression of AD, and the progression of AD would aggravate sleep disturbances. Changes in sleep architecture found in patients with AD occur during the preclinical phase of AD. These changes could be used to detect those most at risk of developing AD. However, this is still only theoretical. While the exact mechanisms and the causal relationship between sleep disturbances and AD remains unclear, these findings already provide a better understanding and offer possibilities to improve targeting of at-risk populations—and the implementation of treatments to curb the cognitive decline of AD patients. Sleep disorder symptoms in psychiatric illnesses Schizophrenia In individuals with psychiatric illnesses sleep disorders may include a variety of clinical symptoms, including but not limited to: excessive daytime sleepiness, difficulty falling asleep, difficulty staying asleep, nightmares, sleep talking, sleepwalking, and poor sleep quality. Sleep disturbances - insomnia, hypersomnia and delayed sleep-phase disorder - are quite prevalent in severe mental illnesses such as psychotic disorders. In those with schizophrenia, sleep disorders contribute to cognitive deficits in learning and memory. Sleep disturbances often occur before the onset of psychosis. Sleep deprivation can also produce hallucinations, delusions and depression. A 2019 study investigated the three above-mentioned sleep disturbances in schizophrenia-spectrum (SCZ) and bipolar (BP) disorders in 617 SCZ individuals, 440 BP individuals, and 173 healthy controls (HC). Sleep disturbances were identified using the Inventory for Depressive Symptoms - clinician rated scale (IDS-C). Results suggested that at least one type of sleep disturbance was reported in 78% of the SCZ population, in 69% individuals with BD, and in 39% of healthy controls. The SCZ group reported the most number of sleep disturbances compared to the BD and HC groups; specifically, hypersomnia was more frequent among individuals with SCZ, and delayed sleep phase disorder was three times more common in the SCZ group compared to the BD group. Insomnias were the most frequently reported sleep disturbance across all three groups. Bipolar disorder One of the main behavioral symptoms of bipolar disorder is abnormal sleep. Studies have suggested that 23-78% of individuals with bipolar disorders consistently report symptoms of excessive time spent sleeping, or hypersomnia. The pathogenesis