BDNF and insomnia: Moreover we found a significant main effect for the independent variables: (i) insomnia severity groups (F = 7.775, p = 0.008) with an incremental decrease in serum BDNF levels from the no insomnia to the sub threshold and to the clinical insomnia group, (ii) score on the PSS (F = 8.230, p = 0.006), with decreased serum BDNF levels associated with increased scores on the PSS and (iii) the covariate smoking (F = 14.154, p<0.001).