Objective Sleep Characteristics and Neoadjuvant Immunotherapy Response in Gastric/GEJ Cancer

Inclusion Criteria:

1. Age greater than 18 years, regardless of sex.
2. Histologically confirmed gastric adenocarcinoma or gastroesophageal junction adenocarcinoma.
3. Locally advanced, resectable disease as assessed by imaging or a multidisciplinary team, based on the 8th edition of the AJCC staging system, typically cT3-4a, any N, or any T with N-positive disease, corresponding to stage II-III disease, without distant metastasis.
4. Scheduled to receive neoadjuvant therapy followed by radical surgery.
5. Eastern Cooperative Oncology Group performance status (ECOG PS) of 0-1.
6. No prior systemic anticancer therapy for the current tumor at study baseline.
7. Willing to participate in the study and able to provide written informed consent.

Exclusion Criteria:

1. Presence of distant metastasis, peritoneal metastasis, or disease considered no longer suitable for curative-intent treatment.
2. Prior neoadjuvant chemotherapy, immunotherapy, radiotherapy, or other systemic anticancer therapy for the current tumor.
3. Severe cognitive impairment, acute psychiatric disorder, or any other condition that prevents the participant from completing study procedures.
4. Current treatment with antidepressants, anxiolytics, sedative-hypnotics, or other psychotropic medications, with any of the following occurring within 4 weeks before enrollment:

   1. Increase or decrease in the dose of the relevant medication by 25% or more from the previous maintenance dose;
   2. Initiation, discontinuation, or replacement of antidepressants, anxiolytics, sedative-hypnotics, or other psychotropic medications;
   3. Adjustment of treatment due to worsening anxiety, depression, insomnia, or other psychiatric or psychological symptoms;
   4. Any medication change or psychological condition judged by the investigator to potentially affect sleep monitoring results or study compliance.
5. Any other condition that, in the opinion of the investigator, makes the participant unsuitable for this study.