Dozing all day even after a full night's sleep? It could be narcolepsy or sleep apnea.

With some sleep disorders, such as sleep apnea, narcolepsy, or periodic limb movement disorder, patients may not realize why or even if their sleep is disturbed throughout the night. They might blame their frequent waking on another issue—bad dreams or a weak bladder, for example—or may not even arouse from slumber long enough to realize they're awake.

Chances are, however, they're not staying asleep for the time it takes to go through the deep, restorative stages of sleep needed to feel refreshed in the morning. That's why, in many cases, how you feel during the day is the best indicator of whether you might have a problem.

"I've learned not to rely on the question, 'How do you sleep,'" says Ralph Downey III, PhD, director of the Sleep Disorders Center at Loma Linda University Medical Center in Loma Linda, Calif. "Because the group that has the real problem—the ones who nod off whenever they're sedentary—they always say they sleep great. Well, it's just that they sleep everywhere, including at work and behind the wheel."

This was the case with Virginia Arguello, 44, a medical transcriptionist in Hayward, Calif. In her mid-30s, Arguello began falling asleep during her morning drive to work, even after a full night's sleep.

"I was scared to get in the car by myself," she says. "I was ashamed because I didn't have energy to play with my kids and I couldn't concentrate at work. But I have a thyroid problem, so I just blamed my sleepiness on that plus the fact that I was raising three daughters and working full time."

Luckily something else tipped off her doctor: Arguello had recurring nightmares of being underwater, unable to swim up and reach the surface. "I would wake up gasping," she says. "My husband complained that I snored and gasped for air."

When she was diagnosed with sleep apnea, Arguello received a continuous positive airway pressure (CPAP) machine to keep her airways from closing while she slept. "After just the first night, I remember the difference being so dramatic, like night and day," she recalls. "I never realized how much energy I'd lost and how sleep deprived I'd become, until I had one truly good night of sleep."