A 78-year-old woman is brought to the clinic by her daughter because she is concerned about her mother's mood. The patient's husband of 48 years passed away 6 months ago after a lengthy illness due to metastatic colon cancer. Since then she reports having a poor appetite, decreased interest in activities, and frequent thoughts about dying. She is started on nortriptyline to help improve her mood and functional status. Which of the following is the most common side effect of nortriptyline?