This prospective observational study will enroll 120 patients with locally advanced gastric or gastroesophageal junction adenocarcinoma who are scheduled to receive neoadjuvant immunotherapy followed by radical surgery. Non-invasive objective sleep monitoring will be performed during the neoadjuvant treatment period to assess sleep characteristics, including sleep duration, sleep efficiency, device-estimated deep sleep proportion, nocturnal awakenings, sleep regularity, heart rate, and heart rate variability. The primary objective is to evaluate the association between objective sleep characteristics and major pathological response (MPR) after neoadjuvant immunotherapy. This study will not alter standard treatment decisions, surgical procedures, or perioperative management.

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.

Adult patients with histologically confirmed locally advanced gastric adenocarcinoma or gastroesophageal junction adenocarcinoma who are assessed as resectable by imaging or a multidisciplinary team and are scheduled to receive neoadjuvant immunotherapy followed by radical surgery. Eligible participants should have stage II-III disease without distant metastasis, an ECOG performance status of 0-1, and no prior systemic anticancer therapy for the current tumor at study baseline.