The principal objective is to evaluate the effect of CPAP treatment on blood pressure (BP) in normotensive patients with nondipper circadian pattern with sleep apnea-hypopnea syndrome (SAHS). The secondary objectives are: i) To evaluate the prevalence of the different circadian patterns of BP in ambulatory blood pressure monitoring (ABPM) ii) To establish the relationship between CPAP compliance and BP response; iii) Identify which ABPM variables and biomarkers are related to BP response; iv) Evaluate changes in the biomarkers' profile after CPAP treatment.

Methodology: Open-label, parallel, prospective, randomized and controlled trial including normotensive patients with nondipper circadian pattern diagnosed with severe SAHS without significant somnolence. 64 patients with nondipper circadian pattern will be included and will be randomized to receive CPAP (32) or conservative treatment (32). ABPM and blood sample collection will be performed at the beginning of the study and at 3 months.

1. Recruitment: Normotensive subjects referred to the sleep unit of the Hospital Santa Maria (Spain) who are diagnosed of severe OSA (AHI≥30) by a sleep study and nondipper circadian blood pattern confirmed by ABPM and who do not present significant somnolence (Epworth≤18) will be included in the study.

2. Definition of the groups: Patients will be randomized to receive one of the following treatments:

   • CPAP: Patients who will receive CPAP treatment. The CPAP titration will be carried out with an automatic or manual CPAP according to usual clinical practice.
   • Conservative treatment: Patients who will receive conservative treatment based on hygienic-dietetic measures.

3. Randomization: It will be carried out with an automated platform

4. Duration of the treatment: All patients randomized to CPAP will begin the treatment as soon as possible after randomization, and will continue treatment until the study ends (3 months).

   Patients who interrupt treatment will be excluded from the study. ABPM and blood sample collection will be performed on all included patients at the beginning of the study and at 3 months under treatment (CPAP or conservative care).

5. Follow-up: All patients will be evaluated at the beginning of the study (T0), at first month (T1) and at three months (T2), during the follow-up period.

Inclusion Criteria:

• Men and women over 18 years of age
• Referred to the sleep unit for suspected OSA
• Being normotensive and presenting an AHI ≥ 30 in the sleep study
• Signature of the informed consent.

Exclusion Criteria:

• Previous CPAP treatment
• Significant somnolence defined by an Epworth Sleepiness Scale (ESS) score higher than 18
• Psychophysical inability to complete questionnaires
• Previous diagnosis or suspicion of another sleep disorder
• Presence of more than 50% of central apneas or Cheyne-Stokes respiration
• Having a serious chronic disease: neoplasia, renal failure, severe chronic obstructive pulmonary disease, chronic depression and other chronic limiting diseases
• Medical history that may interfere with the objectives of the study or, in the opinion of the researcher, may compromise the conclusions
• Any medical, social or geographical factor that may endanger the patient's compliance
• Having a profession of high risk (professional driver).