A three-period parallel (baseline, treatment, and post-treatment) design clinical research was conducted. All participants visited a clinical trial center of Korea University Anam Hospital for baseline measurement of clinical variables including ABPM and hematologic biomarkers (ACTH, aldosterone, norepinephrine, cortisol, and 25-hydroxyvitamin D3). After the baseline assessment, participants visited 'Cholipo beach' (Taean, Republic of Korea) for 3 nights and 4 days for the intervention (sunlight exposure program). After a week of wash-out period, post-treatment measurement was processed at Korea University Anam Hospital. Participants were prohibited from vigorous physical activity, alcohol drinking, and smoking during the study period. A normal-sodium diet (2800 mg/day)[14], caffeine and smoking restriction was suggested during the whole study period. To control the effect of circadian rhythm on endocrine blood tests and ABPM, we fixed sampling and monitoring start time at 10:00 am. All participants received the same measurements as they did in the baseline measurement during treatment and post-treatment periods.

Inclusion Criteria:

• prehypertensive patients
• vitamin D insufficiency
• age 20-45

Exclusion Criteria:

• medical comorbidity other than prehypertension
• participant using medication
• Subjects with a history of cardiovascular, renal, hepatic, gastrointestinal, hematologic, psychological, or gastrointestinal abnormalities, taking any kind of medications