Overall, more than a quarter of our survey population said it took them 30 minutes or longer to fall asleep the previous night, and one quarter awakened in the middle of the night and couldn’t go back to sleep for at least a half-hour. Much of the time, respondents’ sleep was interrupted because they had to use the bathroom. Also, a third woke up much earlier than they had hoped. Such problems weren’t anomalies: Three-quarters of the respondents told us that was a typical night.

Among problem sleepers, 71 percent took at least 30 minutes to nod off; 59 percent woke up in the middle of the night and couldn’t fall back asleep for a half-hour or longer; and 65 percent awoke much too early.

No single reason stood out as a cause of chronic sleeplessness. Our survey uncovered six characteristics that problem sleepers had in common, the most prevalent being high stress levels. Most of the time, respondents were anxious over family or money concerns, health issues, or work woes. Issues most often linked to sleep loss were worries about commuting, personal health, problems with children, and reactions to news events.

Besides stress, problem sleepers had other issues that exacerbated their insomnia. Almost 65 percent suffered from physical pain caused by arthritis or other conditions, 38 percent suffered from a respiratory condition such as asthma or a cold, and 50 percent suffered from mood disorders, including depression.

Other factors associated with sleeplessness are heart disease, Alzheimer's disease, Parkinson's disease, hyperthyroidism, attention deficit hyperactivity disorder, and menopause.

Insomnia can strike at any age, including in childhood. Those 55 and older are more likely to have other illnesses or aches and pains that can disrupt sleep; they’re also more likely to be taking medications that can keep them from falling and staying asleep. Also, as we age our bodies tend to secrete lower amounts of substances that promote deep sleep and regulate our internal body clocks, according to the Mayo Clinic.

Certain activities and behaviors, such as drinking alcohol or caffeinated drinks too close to bedtime, exercising late at night, following an irregular morning and nighttime schedule, shift work, and too much mental stimulation before settling in, can disrupt sleep, too. Traveling, especially shuttling across time zones, can affect sleep. And environmental factors, such as a room that’s too hot, cold, noisy, or bright, can block a good night’s sleep.