A 52-year-old female with major depressive disorder well-controlled on escitalopram 20 mg daily reports persistent sleep-onset and sleep-maintenance insomnia despite good sleep hygiene practices. She wants to avoid benzodiazepines and Z-drugs due to a family history of substance use disorder. Her PMHNP prescribes trazodone 50 mg at bedtime. The patient is surprised that such a low dose of an antidepressant could help her sleep and asks for an explanation. Which of the following best describes the pharmacologic basis for trazodone's hypnotic effect at low doses?