The good news is 72 percent of adults report they do get the sleep they need. On average, American adults sleep 6.8 hours a night and 7.4 hours a night on weekends, according to the poll, conducted for the National Sleep Foundation. Yet while more than three-fourths of the 1,506 people queried say they don't have a sleep problem, an almost identical number claim at least one symptom of a sleep problem a few nights a week or more. There's a disconnect between symptoms and recognition of the link to troubled sleep.

The survey also found that one-half of respondents say they feel tired, fatigued or not up to par at least one day a week, while 17 percent say it happens almost daily.

Still, it may take something as serious as a car crash or an irritability-induced breakup to convince people who are sleep-deprived to get help. It's often a sleeping partner who notices a disruptive sleep pattern and persuades someone to see a doctor.

Sleep deprivation is complex, with many possible causes.

Most often, people simply don't allow enough time to sleep, trying to stretch the day just a bit more to squeeze in one more task. Excessive worry or depression can disrupt sleep. So can a sleep disorder.

People who sleep in noisy surroundings, waking over and over, become sleep deprived. So do those who travel across time zones or do shift work. Illness causing pain or difficulty breathing disrupts sleep.

Poor-quality sleep is usually treatable, say several board-certified sleep specialists, using behavior modification, medications, surgery or other interventions.

If you were to simply ask people about their sleep problems, insomnia seems like the most common one, specialists say.

"Everybody experiences short-term insomnia for a variety of reasons," Dupont says. "If it's chronic, lasting more than six months, that's less common, and those are tough patients."

He estimates doctors successfully treat perhaps half of those who complain of years-long, even decades-long insomnia.

Insomniacs, including folks who can't go to sleep or who can't stay asleep, typically don't seem that tired in the daytime. They may not feel great, but they don't usually doze off. The sleepy folks are the sleep-deprived: shift workers, the many who short themselves on sleep time, or those who have a sleep disorder like sleep apnea. Someone with insomnia tends to be hyper-aroused. Although tired, they don't sleep in the day and they don't sleep well at night, either.

Doctors have a lot to consider in treating a chronic insomniac, such as a psychiatric diagnosis like depression. And it's hard to say if insomnia triggered the depression or if depression jump-started the insomnia. "There clearly are many medical problems that can affect your sleep," Dupont says.

Some insomnias may be caused by post-traumatic stress disorder. Females who have been abused often develop insomnia.

Lifestyle perpetuates insomnia. "As a culture, we don't value sleep enough," Dupont says. "We want to cram in an extra hour of work, and we feel it's worth it. Or an extra hour of studying. Most of us by choice are sleep-deprived."