Interesting article on the effects of the duration of CPAP. Bottom line - more is better up to 7 hrs a night; no improvement after that. Consistent nightly use is a must. Subjective measures of effectiveness (how you think you feel) and objective measures (how you test) may not agree.

From the journal Sleep. 2007;30:711-719. Summarized at Medscape:

Longer Nightly CPAP Duration May Improve Outcomes in Patients With Sleep Apnea

Clinical Context
OSA is associated with an increased risk for both reduced quality of life and cardiovascular morbidity. However, treatment with CPAP has been demonstrated to improve quality of life, neuropsychologic performance, and hypertension among patients with OSA.

Despite the benefits of treatment associated with CPAP, many patients do not use this therapy regularly. Missing treatment for even 1 night can promote a return to daytime sleepiness, worse response performance, and an increase in the apnea hypopnea index. At the same time, there is no consensus regarding the effects of varying numbers of hours per night of CPAP use. The current study by Weaver and colleagues addresses this subject.

Study Highlights
Study subjects were from 7 sleep disorder treatment centers in the United States and Canada. All participants were between the ages of 21 and 60 years, had a diagnosis of OSA, and had an apnea hypopnea index score of 15 or greater. Patients with coexisting pulmonary disease, congestive heart failure, or cerebrovascular accident were excluded from study participation.
Participants underwent a polysomnogram at baseline and received a CPAP device. The CPAP machine recorded its use automatically, and the authors defined the minimum standard of CPAP use to be 20 minutes per day.

The main study outcomes were subjective sleepiness and objective sleepiness, as measured with the Epworth Sleepiness Scale and Multiple Sleep Latency Test, respectively. The authors also measured functional status via the Functional Outcomes of Sleep Questionnaire. These outcomes were measured at baseline and again at 3 months.

149 participants provided data for analysis. 87.2% of subjects were men, and most were white. The average body mass index was 38 kg/m2.
Half of subjects had abnormal pretreatment responses for all 3 of the outcome measures at baseline, but this figure had declined to 10.9% by the end of the study period. 33.8% of participants had normal responses for all 3 outcome measures at the end of the study.

CPAP improved both subjective and objective measures of sleepiness, with the amelioration of the former greater than the latter.

The mean use of CPAP was 4.7 hours per night, and the range of use was 0 to 8.1 hours per night.

Participants who had positive test findings for subjective sleepiness used CPAP for an average of 4 hours per night vs an average of 5.1 hours per night among subjects without posttreatment sleepiness. Overall, there was a negative linear relationship between duration of CPAP use and subjective sleepiness, although approximately 20% of participants with 8 hours of CPAP use per night still reported excessive sleepiness.

Participants whose objective measures of sleepiness improved used CPAP for an average of 1.2 hours longer than those whose objective sleepiness did not improve. Again, there was a negative linear relationship between duration of CPAP use and objective sleepiness.

Longer nightly duration of CPAP use also improved functional status, although the maximum benefit in this outcome was achieved with 7 hours of CPAP use and did not increase with longer durations of therapy.

Examining subgroups based on severity of OSA and other baseline or treatment factors did not significantly alter the main study outcome.

Pearls for Practice
Although CPAP may improve quality of life, neuropsychologic performance, and hypertension among patients with OSA, missing treatment for even 1 night can worsen symptoms.

The current study showed a negative and linear relationship between the duration of nightly CPAP therapy and subjective and objective measures of sleepiness among patients with OSA. Functional status also improved with longer nightly duration of CPAP, although a maximum effect in this outcome was achieved at 7 hours of CPAP therapy.