Q. My son is 11 months old and almost since birth has had what resembles a bruise on his lower back. My neighbor's daughter had a similar mark throughout her first year of life. My pediatrician says not to worry, but I'm still concerned. What causes such marks?

A. Birthmarks come in many shapes and types. It sounds like your son has what's commonly called a Mongolian spot, a flat, slate-gray or gunpowder-blue discoloration usually located at the end of the spine. It's mistakenly named because, although it occurs in Orientals, it's found in all races.

Mongolian spots may be present at birth or appear soon after, and usually enlarge before gradually fading. Most disappear completely by the time the baby turns 1 year old. The cause is unknown, but they aren't a sign of something else wrong.

Another birthmark is a flat, light brown patch called a "cafe' au lait" spot, resembling the color of coffee with milk. They're also harmless, but if many are present (say six or more), they may be a clue to the presence of a hereditary disorder called neurofibromatosis.

Two other birthmarks result from blood vessels going awry. Port wine stains are flat or pebbled patches of purplish red skin, usually occuring on the face or arms. They generally last for life, though they may fade a little. Some people use makeup (like Covermark) to cover this kind of birthmark. There is no satisfactory treatment for it.

"Strawberry" marks are raised, bumpy, dark red growths that usually get larger during the first six to 12 months of life. After that, they gradually clear, usually from the center outward. Most disappear in early childhood, so treatment (with surgery or lasers) is usually not needed.

Q. Although I have no trouble going to sleep, I wake up after a few hours, and have difficulty getting back to sleep. I don't feel like getting out of bed and doing something, and am reluctant to take sleeping pills. I'm 76 years old and have been bothered by this for several years, and it seems to be getting worse. What can I do about this?

A. Insomnia, or being unable to sleep satisfactorily, affects most people from time to time and plagues many people regularly. As you get older, you tend to sleep more lightly and erratically. Age-related changes in sleep may explain your problem.

Many people overestimate the amount of sleep they need and underestimate the amount they get during a restless night. A sign of whether you're getting enough sleep is your ability to function well during the day. If restless nights are followed by sleepy days, you may have a problem that bears looking into.

Worry, anxiety and depression are common causes of insomnia. Painful conditions, such as arthritis, also keep many older people awake. In such cases, pain relievers, not sleeping pills, are what's needed. Because they interfere with normal sleep patterns, sleeping pills and alcohol may actually start a vicious cycle of insomnia, sleeping pill use, and worsening insomnia.

If you don't have an underlying physical or emotional reason for insomnia, you might try the following sleep aids:

Make your sleep routine as regular as possible.

Exercise during the day. Walking is good.

Don't take naps during the day.

Cut back on or stop drinking coffee, tea, colas and other caffeine-containing beverages, especially at night.

Don't use pain relievers like Anacin, Excedrin and Vanquish, which have caffeine added to them. Read labels carefully.

Cut back on or stop drinking beer, wine or liquor, especially at night.

Don't eat large meals shortly before bed.

Drink a warm glass of milk before going to bed. Milk contains tryptophan, a natural sleep aid.

Take a warm bath (not a shower) at bedtime.

Try muscle relaxation exercises while in bed.

If you're having trouble falling asleep, get up and read something relaxing or do something like knitting that doesn't take a lot of concentration until you feel sleepy. Finally, remember that at your age you might get by on less sleep than you used to.

Q. I'm 20 years old, and every now and then I see dark spots moving across my eyes. What causes them? Is this serious? What should I do about it?

A. It sounds like you have what eye doctors call "floaters," specks of debris floating in the clear fluid within the eye. Many people notice these spots from time to time, but for the most part they're a harmless nuisance, not a sign of something wrong with your vision.

When these spots cross your line of sight, they sometimes catch your attention. Because they're actually present within the eye, they move when your eye moves. If you notice them out of the corner of your eye and try to glance at them, they may quickly jump out of view. Nothing can really be done to make them go away, though they tend to settle out of sight.

Sometimes, however, seeing spots in front of your eyes may be a warning sign of serious eye trouble -- a detached retina, for example, or bleeding within the eye.

The retina is the delicate lining containing the nerve cells of vision. With injury, or sometimes for no apparent reason, the retina peels away from the back of the eye, usually producing partial loss of vision and showers of spots or flashes of light. The difference between these serious spots and harmless floaters is that the serious ones occur suddenly, in greater numbers, and are more persistent. If you have these symptoms, you should immediately be examined by an eye physician (ophthalmologist), who can repair a detachment if it's present.