This is a medical question of type : exams and tests 
 How to diagnose Narcolepsy ? 
 It can take as long as 10 to 15 years after the first symptoms appear before narcolepsy is recognized and diagnosed. This is because narcolepsy is fairly rare. Also, many narcolepsy symptoms are like symptoms of other illnesses, such as infections, depression, and sleep disorders.
                
Narcolepsy sometimes is mistaken for learning problems, seizure disorders, or laziness, especially in school-aged children and teens. When narcolepsy symptoms are mild, the disorder is even harder to diagnose.
                
Your doctor will diagnose narcolepsy based on your signs and symptoms, your medical and family histories, a physical exam, and test results.
                
Signs and Symptoms
                
Tell your doctor about any signs and symptoms of narcolepsy that you have. This is important because your doctor may not ask about them during a routine checkup.
                
Your doctor will want to know when you first had signs and symptoms and whether they bother your sleep or daily routine. He or she also will want to know about your sleep habits and how you feel and act during the day.
                
To help answer these questions, you may want to keep a sleep diary for a few weeks. Keep a daily record of how easy it is to fall and stay asleep, how much sleep you get at night, and how alert you feel during the day.
                
For a sample sleep diary, go to the National Heart, Lung, and Blood Institute's "Your Guide to Healthy Sleep."
                
Medical and Family Histories
                
Your doctor may ask whether:
                
You're affected by certain factors that can lead to narcolepsy. Examples of these factors include infections, brain injuries, and autoimmune disorders. Some research suggests that environmental toxins may play a role in triggering narcolepsy.
                
You take medicines and which ones you take. Some medicines can cause daytime sleepiness. Thus, your symptoms may be due to medicine, not narcolepsy.
                
You have symptoms of other sleep disorders that cause daytime sleepiness.
                
You have relatives who have narcolepsy or who have signs or symptoms of the disorder.
                
Physical Exam
                
Your doctor will check you to see whether another condition is causing your symptoms. For example, infections, certain thyroid diseases, drug and alcohol use, and other medical or sleep disorders may cause symptoms similar to those of narcolepsy.
                
Diagnostic Tests
                
Sleep Studies
                
If your doctor thinks you have narcolepsy, he or she will likely suggest that you see a sleep specialist. This specialist may advise you to have sleep studies to find out more about your condition.
                
Sleep studies usually are done at a sleep center. Doctors use the results from two tests to diagnose narcolepsy. These tests are a polysomnogram (PSG) and a multiple sleep latency test (MSLT).
                
Polysomnogram. You usually stay overnight at a sleep center for a PSG. The test records brain activity, eye movements, heart rate, and blood pressure. A PSG can help find out whether you:
                
Fall asleep quickly
                
Go into rapid eye movement (REM) sleep soon after falling asleep
                
Wake up often during the night
                
Multiple sleep latency test. This daytime sleep study measures how sleepy you are. It's often done the day after a PSG. During the test, you're asked to nap for 20minutes every 2 hours throughout the day. (You will nap a total of four or five times.)
                
A technician checks your brain activity during this time. He or she notes how quickly you fall asleep and how long it takes you to reach various stages of sleep.
                
An MSLT finds out how quickly you fall asleep during the day (after a full night's sleep). It also shows whether you go into REM sleep soon after falling asleep.
                
Other Tests
                
Hypocretin test. This test measures the level of hypocretin in the fluid that surrounds your spinal cord. Most people who have narcolepsy have low levels of hypocretin. Hypocretin is a chemical that helps promote wakefulness.
                
To get a sample of spinal cord fluid, a spinal tap (also called a lumbar puncture) is done. For this procedure, your doctor inserts a needle into your lower back area and then withdraws a sample of your spinal fluid.