About 200,000 Americans and 3 million people worldwide have narcolepsy — a condition that causes them to have trouble sleeping at night and suddenly fall asleep during the day.

A smaller number of people with narcolepsy also get extreme muscle weakness when they experience an intense emotion such as laughter or crying.

The sleepiness combined with muscle weakness is called narcolepsy with cataplexy. It’s not dangerous, but the sudden attacks of sleepiness and weakness can have a big impact on your quality of life.

Narcolepsy is a sleep disorder in which your brain has trouble keeping you awake. People with narcolepsy wake up often during the night and are extremely tired during the day. They may have attacks where they suddenly fall asleep in the daytime.

Some people with narcolepsy also have cataplexy — episodes of muscle weakness that are triggered by strong emotions such as laughter, surprise, or crying. Doctors call narcolepsy with cataplexy type 1 narcolepsy.

Cataplexy usually starts a few weeks or months after narcolepsy. But rarely, muscle weakness begins before narcolepsy symptoms.

Cataplexy is sudden muscle weakness or paralysis that happens after you’ve experienced strong emotion. These attacks can vary in severity and length.

Some people have mild attacks where only a few of their muscles become unsteady. Your knees may weaken, your jaw may drop, or your eyelids may droop. The symptoms are sometimes too subtle to notice.

Other people have more severe attacks that cause them to lose all control over their muscles. Their legs may buckle or they can collapse entirely. People often can’t move or speak during one of these attacks, but they’re awake and aware of what’s going on around them.

The muscle twitching during cataplexy can look like a seizure. In fact, it’s sometimes misdiagnosed as a seizure disorder.

Narcolepsy affects signals in your brain that are supposed to keep you awake.

Low levels of the chemical hypocretin cause narcolepsy with cataplexy. This chemical, which is produced in a brain region called the hypothalamus, controls sleep and wakefulness. When it’s in short supply, your brain has trouble regulating your sleep-wake cycles.

The loss of muscle tone, cataplexy, happens because sleep and wakefulness overlap in narcolepsy. It’s the same loss of muscle tone that naturally happens during rapid eye movement (REM) sleep. That loss of muscle tone causes you to lose control over your body when you’re awake.

The lack of hypocretin makes it hard for someone to stay awake during the day, blurring the line between wakefulness and sleep. Some people experience hallucinations and sleep paralysis when they’re falling asleep or waking up.

Certain people with autoimmune disorders are more likely to get narcolepsy with cataplexy. Their immune system mistakenly turns against their body and attacks the brain cells that produce hypocretin.

Less often, the cause of narcolepsy is an injury that damages the brain. In about 10 percent of cases, narcolepsy runs in families.

A cataplexy attack is a sudden bout of muscle weakness triggered by strong emotions such as laughter, anger, or surprise. Attacks of cataplexy usually last about a minute or two and stop on their own. Some people fall asleep afterward.

Less commonly, people have attacks that last up to 30 minutes. Even in more severe attacks, the person remains conscious.

About 14 out of every 100,000 people have narcolepsy with cataplexy. Narcolepsy alone is more common, affecting 65 out of every 100,000 people.

Symptoms usually first appear in the late teens to early 20s. More women are affected than men. Only about 25 percent of people with narcolepsy have a diagnosis and are receiving treatment.

Narcolepsy with cataplexy causes bouts of sleepiness and muscle weakness during the day. Most people who have this condition aren’t getting the treatment they need to control their symptoms.

If you find that you constantly fall asleep during the day and your muscles get weak when you experience strong emotions, see your doctor. Narcolepsy can take some time to diagnose. Once you do get a diagnosis, treatments are available to help you avoid symptom attacks.