GFAP and Alzheimer disease: Another biomarker of relevance is NfL, levels of which increase in CSF of patients with AD and track well with cortical thinning and thus also with clinical progression of AD.10 These changes also parallel changes in blood.10 Given that we observed no sleep loss–induced changes in NfL (the most well-established neuronal injury marker) or GFAP, this argues against the possibility that acute sleep loss would also result in acute neuroaxonal injury.