The claim that tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs) cause sedation by increasing the levels of serotonin and norepinephrine is inaccurate for both drug classes. The mechanism for sedation is different for TCAs, and sedation is not a primary effect for most SSRIs. In fact, increased serotonin and norepinephrine are generally associated with wakefulness and arousal.
Location: MRCPsych Paper A Revision Course » 4.3 Pharmacodynamics » 4.3.4 Adverse actions of major psychotropic/psychoactive drug classes