Of the various sleep disorders, the most widespread is insomnia—difficulty in falling or staying asleep. insomnia may be due to a particular situation, such as the breakup of a relationship, concern about a test score, or the loss of a job. often, however, insomnia has no obvious cause. some people are simply unable to fall asleep easily, or they go right to sleep but then wake up frequently. insomnia afflicts about a quarter of the population of the united states. interestingly, many who think they have insomnia are mistaken. observers find that patients who enter sleep laboratories for treatment may sleep much more than they think they do. for example, researchers report that some people who claim to be up all night actually fall asleep in thirty minutes and stay asleep all night. furthermore, some people can accurately recall sounds they heard while asleep. as a result, they believe that they were awake during the night. in many cases, just becoming aware of how long they really do sleep—and understanding that the older they become, the less sleep they need—is enough to “cure” people’s perception that they have a sleep disorder. in fact, some researchers believe the twenty-first century will lead to drugs that act by changing people’s perceptions of how much they have slept, rather than making them sleep more. other sleep problems are less common than insomnia and perceived insomnia, although they are still widespread. for instance, some 20 million people suffer from sleep apnea, a condition in which a person has difficulty breathing and sleeping simultaneously. the result is disturbed, fitful sleep, as the person is constantly reawakened when the lack of oxygen becomes great enough to trigger a waking response. in some cases, people with sleep apnea wake as many as 500 times during the course of a night, although they may not even be aware that they have wakened. not surprisingly, such disturbed sleep results in complaints of fatigue the next day. sleep apnea may also account for sudden infant death syndrome, a mysterious killer of seemingly normal infants who die while sleeping. narcolepsy is an uncontrollable need to sleep for short periods throughout the day. people with narcolepsy go directly from wakefulness to sleep. no matter what the activity—holding a heated conversation, exercising, even driving—the narcoleptic will suddenly slip into sleep. the causes of narcolepsy are not known, although it appears to run in some families. 1. which sentence best states the main idea of the passage? a. insomnia and sleep apnea are disorders that need further scientific study. b. there are several kinds of sleep disorders. c. insomnia may be due to a particular situation or may have no obvious cause. d. many people who think that they have sleep problems may be mistaken.