BDNF and mood disorder: Given the diurnal oscillation pattern of both Bdnf and TrkB (Bova et al., 1998) and their important role in sleep homeostasis (Faraguna et al., 2008) and circadian regulation (Liang et al., 2000), impaired Bdnf-TrkB signaling may promote development of mood disorders by disturbing the normal circadian cycle.