If I lie down during the day, I sleep, for two hours. Can a person learn to take shorter naps?

J.J., Littleton

Well, you could certainly take shorter naps by setting an alarm clock. But, that you spontaneously sleep so long during the day could be a sign that you're not getting enough sleep at night or, possibly, of a sleep disorder that may need medical attention.

A short (20-minute) nap early in the afternoon can be restorative, sleep experts say, because it fits with a natural ''dip" in the circadian cycle -- a time when most of us get a little sleepy anyway. But sleeping longer than that or later in the day may cause problems if you tend to have insomnia at night, said Dr. John Winkelman, medical director of the sleep health center at Brigham and Women's Hospital.

In general, if you get sufficient sleep at night -- at least seven hours -- you shouldn't feel sleepy the next day.

Still, many people, from Winston Churchill to Ronald Reagan, have sworn by regular, short naps, according to the National Sleep Foundation in Washington. A foundation poll taken in 2000 showed that one-third of respondents said they would nap at work if it were allowed; an earlier poll showed that 38 percent of adults nap at least once during the work week, usually for an hour.

But sleeping that long may make you feel worse, said Dr. David Neubauer, associate director of the Johns Hopkins Sleep Disorders Center. After a longer nap, it may take several hours to feel fully alert.

Sleeping a lot during the day also can be a sign of a medical disorder, including sleep apnea, in which you wake up over and over struggling for breath, without knowing it. If you fall asleep uncontrollably during the day, you might have a rare but complex neurological disorder called narcolepsy that also needs treatment.

JUDY FOREMAN