Orexin secretion abnormality involved in excessive somnolence in CNS lymphoma without hypothalamic lesions.

•Hypothalamic lesions cause excessive somnolence due to decreased orexin secretion.•Excessive somnolence may occur even in cases without a hypothalamic lesion.•Decreased CSF orexin levels suggest the involvement of hypothalamic impairment.•CSF orexin is a useful marker for assessing potential hypothalamic somnolence.