Narcolepsy is a chronic sleep disorder characterized by overwhelming daytime drowsiness and sudden attacks of sleep. People with narcolepsy often find it difficult to stay awake for long periods of time, regardless of the circumstances. Narcolepsy can cause serious disruptions in your daily routine.

Contrary to what some people believe, narcolepsy isn't related to depression, seizure disorders, fainting, simple lack of sleep or other conditions that may cause abnormal sleep patterns.

Narcolepsy is a chronic condition for which there's no cure. However, medications and lifestyle changes can help you manage the symptoms. Support from others — family, friends, employer, teachers — can help you cope with narcolepsy.

Nail fungus is a fungal infection in one or more of your nails. An infection with nail fungus may begin as a white or yellow spot under the tip of your fingernail or toenail. As the nail fungus spreads deeper into your nail, it may cause your nail to discolor, thicken and develop crumbling edges — an unsightly and potentially painful problem.

An infection with nail fungus may be difficult to treat, and it may recur. But medications are available to help clear up nail fungus.

Neck pain is a common complaint. Neck muscles can be strained from poor posture — whether it's leaning into your computer at work or hunching over your workbench at home. Wear-and-tear arthritis also is a common cause of neck pain.

Rarely, neck pain can be a symptom of a more serious problem. Seek medical care if your neck pain is accompanied by numbness or loss of strength in your arms or hands or if you're experiencing shooting pain into your shoulder or down your arm.

Nephrotic syndrome is a kidney disorder that causes your body to excrete too much protein in your urine.

Nephrotic syndrome is usually caused by damage to the clusters of small blood vessels in your kidneys that filter waste and excess water from your blood. Nephrotic syndrome causes swelling (edema), particularly in your feet and ankles, and increases the risk of other health problems.

Treatment for nephrotic syndrome includes treating the underlying condition that's causing it and taking medications. Nephrotic syndrome can increase your risk of infections and blood clots. Your doctor may recommend steps to prevent these and other complications of nephrotic syndrome.

Neurodermatitis starts with a patch of itchy skin, but scratching it makes the area even itchier. Eventually you may scratch simply out of habit. This cycle of chronic itching and scratching can cause the affected skin to become thick and leathery.

Neurodermatitis — also known as lichen simplex chronicus — isn't serious, but breaking the itch-scratch cycle is challenging.

Successful treatment of neurodermatitis depends on identifying and eliminating factors that may be aggravating the problem. Over-the-counter and prescription creams can help ease neurodermatitis symptoms.

Neuroblastoma is a cancer that develops from immature nerve cells found in several areas of the body. Neuroblastoma most commonly arises in and around the adrenal glands, which have similar origins to nerve cells and sit atop the kidneys. However, neuroblastoma can also develop in other areas of the abdomen and in the chest, neck and near the spine, where groups of nerve cells exist.

Neuroblastoma most commonly affects children age 5 or younger, though it may rarely occur in older children.

Some forms of neuroblastoma go away on their own, while others may require multiple treatments. Your child's neuroblastoma treatment options will depend on several factors.

Nickel allergy is one of the most common causes of allergic contact dermatitis — an itchy rash that appears when your skin touches a usually harmless substance.

Nickel allergy is commonly associated with earrings and other jewelry, particularly jewelry associated with body piercings. But nickel can be found in many everyday items — from coins to zippers, from cellphones to eyeglass frames.

Nickel allergy can affect people of all ages. A nickel allergy usually develops after repeated or prolonged exposure to items containing nickel. Treatments can reduce the symptoms of nickel allergy. Once you develop nickel allergy, however, you will always be sensitive to the metal and need to avoid contact.