Jack (not his real name) is an airport baggage screener who works a midnight-to-8 a.m. shift so he can take care of family matters during the day. He loves the flexibility of being able to take care of his son when he's off, but admits his energy and alertness have suffered as a result.

"I'm always a cup of coffee away from joining the rest of society," says Jack. Off-duty, that has meant feeling constantly sluggish and sometimes nodding off at the wheel.

At work, although he puts in his best effort, he tends to feel more irritable, looks forward to his break more often, and tries to get passengers through the line at a faster rate. "I guess the quantity of work that a person can do during those hours is less."

Jack is not alone. According to National Sleep Foundation (NSF) polls, half of American employees report that sleepiness on the job interferes with their performance at work. Twenty-six percent would nap during work breaks if their company allowed naps. Not only that, but the line between work and home is blurring. Fifty-eight percent bring work home at night to complete. And those who work long hours report greater impatience, lower productivity, and difficulty concentrating.

Jack is part of a group particularly hard hit by sleep problems: shift workers who labor while most people are asleep and try to snooze while everyone else is awake.

The National Sleep Foundation puts the number of people who fit this description at 22 million Americans, and that figure is reportedly increasing each year. That's because there are more occupations that require around-the-clock attention, besides the trades of law enforcement, health care, energy, and manufacturing, which traditionally have had rotating schedules.

"Our whole society has moved toward a 24/7 kind of economy," says Mark Rosekind, PhD, president and chief scientist of Alertness Solutions, a scientific consulting firm that deals with sleep issues. He says people are now working all the time in retail, banking, information technology, and the media.

Plus, Rosekind says shift workers also could include business travelers who often cross time zones and people who get up very early in the morning to make a one-hour-plus commute to work.

Experts say shift workers are often hardest hit by sleep problems. Their biological clocks are confused, and when they try to get shuteye while the rest of the world is active, they can get disturbed by neighborhood noises, phone calls, kids at home, or a ringing doorbell.

Meir Kryger, MD, professor of medicine at the University of Manitoba in Winnipeg, Canada, says people who don't sleep well can exhibit the following symptoms: