Disturbances of sleep quality, timing and structure and their relationship with other neuropsychiatric symptoms in Alzheimer’s disease and schizophrenia: Insights from studies in patient populations and animal models

•Sleep disturbances are common in Alzheimer’s disease and schizophrenia.•Sleep disturbances can precede Alzheimer’s disease and schizophrenia.•Some sleep disturbances associate with neuropsychiatric symptoms.•Preclinical models can provide insights into mechanisms linking sleep and symptoms.•Standardising methods across disorders aids understanding sleep-symptom relations.