BDNF and major depressive disorder: Evidence indicates that BDNF could play a role in this association: i) in animal studies BDNF levels decreased after chronic stress [10], [11], [12], [13], ii) serum BDNF levels decreased in stress-related major depressive disorder [14], [15], [16], iii) BDNF plays a role in sleep homeostasis [17], [18], [19].