The goal of this clinical trial is to explore whether subanesthetic doses of esketamine can improve the sleep quality of patients undergoing modified radical mastectomy for breast cancer. In this study, 184 people are expected to participate from admission to three days after surgery. The process requires participants to cooperate with the completion of the digital rating scale, the Athens Insomnia Scale, the Visual Analog Pain Rating Scale, the Hospital Anxiety and Depression Rating Scale, and the collection of venous blood for research indicators (IL-6, TNF-α, cor, BDNF). If participants agree to participate in this study, the investigators will assign a unique identification number to each participant and create a medical record for participants. One day before surgery, the investigators will conduct assessments using the Athens Insomnia Scale, the Visual Analog Pain Rating Scale, and the Hospital Anxiety and Depression Scale, and collect approximately 6 mL of venous blood. One day after surgery, the investigators will repeat the assessments using the Athens Insomnia Scale, the Visual Analog Pain Rating Scale, and the Hospital Anxiety and Depression Scale. Additionally, the investigators will collect another 6 mL of venous blood and monitor the use of analgesics. The investigators will follow up with participants three days after the surgery. During this follow-up, investigators will assess any sleep disturbances, administer the Hospital Anxiety and Depression Scale, and review the use of analgesics.