A 49-year-old male with major depressive disorder and comorbid insomnia has been taking trazodone 50 mg at bedtime for 3 months, prescribed by his primary care provider. His sleep has improved significantly, but he continues to endorse persistent depressed mood, anhedonia, poor concentration, and feelings of worthlessness (PHQ-9 score of 16). At his PMHNP evaluation, he states: 'I thought this antidepressant was supposed to help my depression too, why isn't it working?' Which of the following best explains why this patient's depression has not improved on trazodone 50 mg?