ISWD has various causes, including neurological disorders such as dementia (particularly Alzheimer's Disease), brain damage, or intellectual disabilities. It is thought that sufferers have a weak circadian clock. The risk for the disorder increases with age, but only due to increased prevalence of co-morbid medical disorders.
Irregular sleep–wake rhythm
Excessive daytime sleepiness
Non-24-hour sleep–wake disorder
Sexual anhedonia
Dyssomnia
Musical anhedonia
Disordered eating
Sleep state misperception