The goal of this observational study is to investigate the relationship of CLOCK 3111T/C (rs1801260) gene variant with nutritional habits, nutritional status, chronotype, sleep quality, some biochemical parameters in overweight or obese individuals and to observe its effect on weight loss diet intervention. The main questions it aims to answer are:

• Is the frequency of carrying the CLOCK 3111T/C (rs1801260) gene risk allele different between individuals with normal body weight and those who are overweight or obese?
• Do those with the CLOCK 3111T/C (rs1801260) risk allele have a higher frequency of evening chronotype and a shorter sleep duration?
• Is the effect of CLOCK 3111T/C (rs1801260) gene alleles different on weight loss diet response in overweight or obese individuals? A questionnaire will be applied to the individuals in both groups (normal body weight and overweight/obese individuals) to evaluate their general characteristics, eating habits, adherence to Mediterranean diet, DASH and MIND. In addition, anthropometric measurements, 3-day food consumption record and 3-day physical activity record will be taken from individuals. CLOCK 3111T/C (rs1801260) gene variant analysis in whole blood, adiponectin and leptin hormones in serum samples will be studied. Participants those who are overweight/obese will be asked to follow a weight loss diet for 3 months.

Researchers will compare participants with normal body weight and those who are overweight or obese to see if there is a difference between the frequency of carrying the CLOCK 3111T/C (rs1801260) gene risk allele.

Inclusion Criteria:

• Be in the 20-50 age range
• BMI in the range of 25-40 kg/m2 for the study group
• BMI of 18.5-24.9 kg/m2 for the control group

Exclusion Criteria:

• Those with type 1 diabetes
• Those with Type 2 diabetes who take insulin therapy or use oral diabetes medication
• Those who use drugs for weight loss
• Those with unstable cardiovascular disease
• Those with kidney or liver failure
• Presence of diagnosed malabsorption (celiac disease, Crohn's, ulcerative colitis)
• Those who have undergone bariatric surgery
• Those who are pregnant or breastfeeding
• Menopausal women
• Those who use corticosteroids
• Those with severe endocrine disorders (hypothyroidism, hyperthyroidism, hypopituitarism)
• Those with psychiatric or neurological disease
• Weight change of >5% in the last 6 months
• Those with malignant tumors
• Those with eating disorders
• Shift workers
• Relatives
• People from different ethnic backgrounds

The population of the study consists of overweight or obese individuals who applied to Hacettepe University Adult Hospital Endocrinology outpatient clinic. Individuals will be included in the study after being evaluated by the responsible physician and dietitian in Hacettepe University Adult Hospital Endocrinology outpatient clinic in terms of inclusion and exclusion criteria.