This clinical study aims to evaluate the effect of salivary melatonin levels on postoperative pain and sleep quality in patients undergoing root canal treatment at different times of the day. Each participant will receive endodontic treatment on two asymptomatic mandibular molars, one in the morning and the other in the afternoon. Pain intensity and sleep quality will be assessed using standard questionnaires, and salivary melatonin levels will be analyzed.

Postoperative pain following endodontic treatment is a common clinical problem that adversely affects patients' quality of life. Studies have shown that this pain may not only be associated with local tissue trauma and inflammation but can also vary depending on the individual's neuroendocrine system and biological clock (Alhassani & AlGhamdi, 2021).

Many physiological processes in the human body are regulated by the circadian rhythm, which directly influences pain perception, inflammatory responses, and hormone secretion (Smolensky et al., 2015). One of the primary regulatory hormones of circadian rhythm is melatonin, which is secreted mainly by the pineal gland as well as other tissues, reaching peak levels particularly during nighttime. Melatonin is known for its analgesic, anti-inflammatory, antiproliferative, and immunomodulatory properties (Reiter et al., 2010).

In recent years, the effects of melatonin in dentistry have been investigated in the contexts of periodontal regeneration, oral mucosal healing, and bone regeneration (Cutando et al., 2007; Dominguez-Rodriguez et al., 2010). However, clinical studies exploring the role of melatonin during endodontic treatment and its influence on postoperative pain remain very limited. Similarly, no studies in the literature have specifically examined the relationship between melatonin levels and sleep quality in the context of endodontic therapy.

Moreover, some research suggests that dental procedures performed at different times of the day may have varying effects on postoperative pain intensity and the healing process (Segura-Egea et al., 2008). In this regard, understanding how endodontic treatment interacts with systemic biological processes when performed at different times of day is clinically significant.

This study will analyze salivary melatonin levels obtained prior to morning and afternoon endodontic procedures performed on the same patient and will subsequently evaluate their relationship with postoperative pain levels and sleep quality. In doing so, it will provide some of the first clinical evidence regarding the impact of circadian rhythm and melatonin on pain and sleep following endodontic treatment.

In this way, the study not only offers an original approach that may guide clinical practice but also has the potential to lay the groundwork for a personalized dental treatment concept aligned with biological rhythms.

Inclusion Criteria:

Patients aged 25-45 years Patients with two asymptomatic, devital mandibular molars in the same arch No radiographic evidence of periapical lesions or pathological formations in the teeth Patients without systemic diseases (ASA I) Patients who have not used antibiotics, analgesics, or melatonin-containing medications within the last week Patients who have signed the informed consent form

Exclusion Criteria:

Patients with a diagnosis of sleep disorder or those regularly using sleep medications/sedatives Patients with systemic diseases (e.g., hypertension, diabetes, endocrine disorders, etc.) Patients receiving psychiatric treatment Pregnant or breastfeeding patients Patients unable to cooperate during the treatment process Patients with restorative or periodontal complications in their teeth Patients working in a shift system

Individual participant data (IPD) will not be shared because this is a single-center academic thesis study with a limited number of patients. Data include personal health information, and sharing outside the study team is restricted due to ethical and confidentiality considerations.

This is an open-label study; both participants and investigators are aware of the timing of the intervention.