During sleep, some people stop breathing for short periods, a condition known as sleep apnea. This partly awakens them and prevents them from achieving normal, restful sleep. Many people with sleep apnea feel bad, have difficulty thinking clearly, and suffer from daytime sleepiness. These symptoms can be troublesome, or even dangerous (if, for example, the affected person falls asleep while driving a car). The usual treatment for this problem involves wearing a special mask during sleep that uses air pressure to keep the breathing passages open (continuous positive airway pressure, or CPAP). This mask can reduce or eliminate the daytime symptoms, but the treatment is expensive and often annoying. Some people with sleep apnea do not complain of sleep-related symptoms, but they may still have trouble thinking or tend to fall asleep even though they are not aware of those problems. It has been suggested that all patients with sleep apnea be treated with CPAP, whether or not they complain of daytime sleepiness. It is not known, however, whether CPAP treatment actually benefits patients without daytime sleepiness.