The general objective is to evaluate the effect of continuous positive airway pressure (CPAP) treatment in patients with masked hypertension.

The secondary objectives are: i) To evaluate the prevalence of different circadian patterns of BP in ambulatory blood pressure monitoring (ABPM) (dipper / non-dipper); ii) To assess in these patients the relationship between compliance with CPAP treatment and the response to nocturnal BP; iii) To identify variables of the ABPM, and biomarkers that are related to the unfavourable pattern of nocturnal BP response in these patients treated with CPAP; iv) To evaluate the change in the profile of biomarkers with the treatment.

Methodology: Open, parallel, prospective, randomized and controlled study in which an ABPM will be performed in individuals with masked hypertension referred to the sleep unit and diagnosed of OSA (AHI≥ 30) without sleepiness (Epworth≤18).

A total of 64 subjects with OSA and masked hypertension will be recruited. It will be collected blood for the determination of biomarkers. Subsequently, they will be randomized to receive treatment with CPAP (32) or conservative treatment (32). After 3 months of initiation, ABPM and biological determinations will be repeated.

Recruitment: It will be recruited subjects with masked hypertension. For that, normotensive subjects (Blood pressure (BP)<140/90mmHg in the office) referred to the sleep unit of the Hospital Santa Maria (Spain) who are diagnosed of severe OSA (AHI≥30) by a sleep study and who do not present significant somnolence (Epworth≤18) will be proposed to participate in the study. Then, it will be performed an ABPM during-24 hours, and those subjects with an abnormal ABPM results and normal BP in the office will be included, are those subjects called masked hypertensive.

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

1. 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.
2. Conservative treatment: Patients who will receive conservative treatment based on hygienic-dietetic measures.

Randomization: It will be carried out with an automated platform

Duration of the treatment: All patients randomized to CPAP will begin 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).

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

Inclusion Criteria:

1. Men and women over 18 years of age
2. Referred to the sleep unit for suspected OSA
3. Being masked hypertensive and presenting an AHI≥ 30 in the sleep study
4. Signature of the informed consent.

Exclusion Criteria:

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