Many sleep apnea patients may have impaired attention and decision-making ability, even if they are being treated with CPAP, a small study suggests.

Researchers found that people using continuous positive airway pressure (CPAP) to treat apnea, but who still complained of daytime sleepiness, also had problems with certain thinking skills.

"These individuals do not respond to treatment, and eventually will drop CPAP because they do not feel better," said lead study author Ksdy Werli of the Federal University of Sao Paulo in Brazil.

"We chose to study this issue aiming to understand the real difficulties of these patients," Werli said by email.

More on this...

More than 18 million U.S. adults have sleep apnea, a condition that causes people to wake up repeatedly throughout the night as their airways collapse and they cannot breathe, according to the National Sleep Foundation.

One of the most common symptoms of untreated sleep apnea is daytime sleepiness as a result of sleep that is disrupted dozens of times a night and possibly of oxygen deprivation. CPAP is a standard treatment designed to keep airways open to allow a sleeper to breathe normally.

Even when CPAP works well, though, some patients continue to experience excessive sleepiness during the day, Werli's team writes in the journal Sleep Medicine.

Between 6 percent and 34 percent of patients continue to experience daytime sleepiness after treatment and experts don't know why this occurs, Werli told Reuters Health.

To understand what other problems this might cause, the study team collected data on 15 patients, ranging in age from 35 to 60 years old, who had been treated with CPAP but still felt sleepy during the day.

The study team also recruited a comparison group of 15 people with sleep apnea using CPAP and reporting no lingering sleepiness.

The research team tested participants to assess their brain function, including attention, memory and judgment. They also screened participants for depression symptoms because depression can negatively affect mental functions like memory and motivation.

Overall, the people complaining of sleepiness had higher average scores on depression tests. But there was little difference between groups on long-term memory tests.

The people experiencing sleepiness had the greatest trouble with so-called executive functions, which can include attention, motivation, problem-solving and planning.

In particular, these participants struggled problem-solving skills and with a verbal fluency test that measures strategic thinking and organization of words. They did not display differences from the comparison group in other functions including attention and number memory.

Researchers found that having more depression symptoms was linked to poorer performance on verbal fluency tests, but depression didn't explain any of the other results.

Werli noted that the brain functions affected by sleepiness like judgment, criticism and planning are important for helping people adapt to their environment and deal with new situations.

If you have sleep apnea and were effectively treated but you are still sleepy, "you may be at risk for accidents and lower mental performance," Werli said.

This reduced brain function negatively impacts all aspects of daily living, including work, driving performance, social life and relationships, said Renaud Tamisier of Grenoble Alps University in France, who was not involved in the study.

"Sleepiness is really a symptom people do not want to live with," Tamisier told Reuters Health by email.

It is important to identify diseases that affect quality of sleep, but people should also examine their lifestyle to find ways to improve their sleep, Tamisier added.

If the treatment of a sleep disorder is not enough, people can try using sleep hygiene techniques like avoiding napping during the day and not consuming caffeine to close to bedtime, Werli said.

"If a patient has excessive daytime sleepiness and has not yet been diagnosed with a sleep disorder, they should see a doctor to investigate the causes of drowsiness," Werli said.