Depending upon the underlying cause, insomnia is regarded as either "primary" or "secondary." Li explains that primary insomnia, whose occurrence is linked to heredity and body type, is regarded as an illness in its own right. Starting in childhood, some people just have problems sleeping. But most insomnia is secondary insomnia, which is a symptom of other underlying causes.
失眠依據不同的潛在原因，分為「原發性失眠」與「續發性失眠」兩種，李宇宙指出，原發性失眠被視為單純、獨立的疾病，與遺傳體質有關，許多人從幼年期開始就不易入睡。