According to the SEPAR regulation, patients treated with CPAP should be evaluated by the pulmonologist one month after beginning treatment, every 3 months during the first year, every 6 months during the second year and annually from the third year.

To compare the control of side effects between both arms using the percentage of patients who have any of these effects during the follow-up.

To compare the efficacy of the treatment using the change of the Epworth Sleepiness Scale score in both arms and between them.

This scale is a self-administered questionnaire and patients must rate their usual chances of dozing off or falling asleep while engaged in eight different daily activities. The possible answers for each situation and their scores are: 0 = would never doze; 1 = slight chance of dozing; 2 = moderate chance of dozing; 3= high chance of dozing. The Epworth Sleepiness Scale score is the sum of these 8 items scores and it can be interpreted as follows: 0-5 = lower normal daytime sleepiness; 6-10 = higher normal daytime sleepiness; 11-12 = mild excessive daytime sleepiness; 13-15 = moderate excessive daytime sleepiness; 16-24 = severe excessive daytime sleepiness.

To compare the efficacy of the treatment in both arms and between them using the change in the frequency of the snoring according to four levels: never, sometimes, frequently and always or almost always

To compare the efficacy of the treatment in both arms and between them using the change in the frequency of the witnessed apneas according to four levels: never, sometimes, frequently and always or almost always

To compare the efficacy of the treatment in both arms and between them using the change in the frequency of the refreshing sleep according to four levels: never, sometimes, frequently and always or almost always

To compare the intensity of the air leak and residual AHI measured by the CPAP device between both arms.

To compare the change of quality of life before and after the follow-up in both arms and between them, using the change in the EuroQol-5 dimensions-3 levels questionnaire. This is a standardised measure of health status that comprises a descriptive system and a visual analogue scale. The first comprises 5 dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension has 3 levels: 1 = no problems; 2 = some or moderate problems; 3 = extreme problems. The health state is defined by combining one level from each dimension (5 digit code). This descriptive system is converted into a single index value facilitating comparisons and the calculation of quality-adjusted life years (QALYs). The visual analogue scale records the respondent's self-rated health on a 20 centimeters vertical, visual analogue scale with endpoints labeled as: 100 = the best health you can imagine and 0. = the worst health you can imagine.

To compare the effect of the treatment in the quality of sleep using the change in the rhythm of distal skin temperature by ambulatory circadian monitoring including this variable. The investigators will compare the rhythm of this variable before and after CPAP treatment in both arms and the difference between them. For this propose, all randomized patients will carry a watch-like device for ambulatory circadian monitoring during one week before starting CPAP treatment and during the last week of the six months of follow-up and treatment with CPAP.

To compare the effect of the treatment in the quality of sleep using the change in the rhythm of activity by ambulatory circadian monitoring including this variable. The investigators will compare the rhythm of this variable before and after CPAP treatment in both arms and the difference between them. For this propose, all randomized patients will carry a watch-like device for ambulatory circadian monitoring during one week before starting CPAP treatment and during the last week of the six months of follow-up and treatment with CPAP.

To compare the effect of the treatment in the quality of sleep using the change in the rhythm of exposure to light by ambulatory circadian monitoring including this variable. The investigators will compare the rhythm of this variable before and after CPAP treatment in both arms and the difference between them. For this propose, all randomized patients will carry a watch-like device for ambulatory circadian monitoring during one week before starting CPAP treatment and during the last week of the six months of follow-up and treatment with CPAP.

To compare the effect of the treatment in the quality of sleep using the change in the result of an integrated analysis of the distal skin temperature, activity and position records obtained by ambulatory circadian monitoring including these variables. The researchers will compare this result before and after CPAP treatment in both arms and the difference between them. For this propose, all randomized patients will carry a watch-like device for ambulatory circadian monitoring during one week before starting CPAP treatment and during the last week of the six months of follow-up and treatment with CPAP.

To compare the efficacy of the treatment according to the presence or not of a couple, comparing the percentage of patients with CPAP compliance ≥4 hours per day between both groups of patients (all patients with and without couple), within each arm (control and intervention arm) and between arms.

To analyze the influence of the patients' age on the percentage of patients with a CPAP compliance ≥4 hours per day in both arms.

To compare the efficacy of the treatment depending on the gender, comparing the percentage of patients with a CPAP compliance ≥4 hours per day between all men and all women, within each arm and between arms.

To analyze the effect of the CPAP pressure on the percentage of patients with a CPAP compliance ≥4 hours per day in both arms.

To compare the efficacy of the treatment depending on the type of mask used during the CPAP treatment, comparing the percentage of patients with a CPAP compliance ≥4 hours per day in the group of patients who use nasal pillow, full face and nasal mask in both arms and between them.

To compare the efficacy of the treatment depending on the use of humidifier during the CPAP treatment, comparing the percentage of patients with a CPAP compliance ≥4 hours per day in the group of patients who use or not humidifier in both arms and between them.

To compare the efficacy of the treatment depending on the presence of any side effect during the CPAP treatment, comparing the percentage of patients with a CPAP compliance ≥4 hours per day in the group of patients with and without any side effect in both arms and between them.

The researchers will perform a cost-effectiveness analysis using the average compliance as main variable of effectiveness, and the EuroQol-5 dimensions-3 levels descriptive system to calculate QALYs.

EuroQol-5 dimensions-3 levels descriptive system is a standardised measure of health status that comprises 5 dimensions: mobility, self-care, usual activities, pain/discomfort and anxiety/depression. Each dimension has 3 levels: 1 = no problems; 2 = some or moderate problems; 3 = extreme problems. The respondent have to indicate his/her health state according to the most appropiate statement in each dimension. The health state is defined by combining one level from each of the 5 dimensions (5 digit code) and then, this descriptive system is converted into a single index value facilitating comparisons and the calculation of quality-adjusted life years (QALYs).