Methodological differences are likely to explain part of this heterogeneity, as various methods have been used to assess sleep (i.e. questionnaires, actigraphy, and polysomnography), in different study populations (i.e. patients with dementia, MCI, and cognitively normal older adults) and sometimes very small samples (i.e. ≤10 APOE4 carriers in previous studies using polysomnography, except one study including 555 male APOE4 carriers, but no women [25]). The gene discussed is APOE; the disease is dementia.