NTF4 and insomnia: The neurobiological underpinnings of both OSA and insomnia involve complex interactions among various neuromodulators, including brain-derived neurotrophic factor (BDNF), its precursor proBDNF, glial-cell-line-derived neurotrophic factor (GDNF), neurotrophin-3 (NTF3), and neurotrophin-4 (NTF4) [13,14,15,16,17,18].