There are five common symptoms of narcolepsy, but very few people have all of them.
The first and most important is sudden daytime sleepiness, so extreme that it becomes impossible to stay awake for more than a few moments no matter how hard the person tries, even if they're standing up. This is most likely to occur when the person is passive or bored - for example, in the classroom or at work. In a few cases, however, it occurs during physical exertion or stress.
It's not difficult to rouse someone from such a state, and upon waking people may feel refreshed, but the tiredness may return very soon, forcing another bout of sleep. This may happen several times a day in the most severe cases. If someone with narcolepsy is left undisturbed, the sleep may last from a few minutes to a few hours.
Despite this, people with narcolepsy do not actually tend to sleep more hours in a day than people without the condition. This naturally implies that people with this condition get less sleep at night, and indeed many complain of unrefreshing nighttime sleep that's often interrupted by nightmares.
As well as briefly falling asleep several times during the day, people with narcolepsy are prone to waking up several times a night.
The other three symptoms of narcolepsy (cataplexy, sleep paralysis, and Hypnagogic phenomena) are more rare, but most sufferers experience at least one of them from time to time. They all occur in fully conscious people and can be frightening if the person hasn't yet been diagnosed and doesn't know he or she has narcolepsy.
In cataplexy, the low muscle tone of REM sleep appears suddenly in a person who is fully awake at a time of strong emotion. Laughter, anger, fear, happiness, or, most often, a simple surprise can cause a person to go suddenly limp and drop to the ground.
Sleep paralysis occurs just before going to sleep or just after waking up. A temporary but complete paralysis prevents the person from moving for a few seconds. Sleep paralysis isn't actually that rare, especially in children, but it's only a sign of narcolepsy if one also experiences the primary symptom of suddenly falling asleep in the daytime.
Hypnagogic phenomena (hallucinations) also occur just before drifting off to sleep. Hypnopompic phenomena are hallucinations that occur just after waking, but are even rarer. Basically, they are vivid dreams that project into the waking period. Because they occur in people who aren't actually asleep, these dreams are sometimes called hallucinations.
There is one potential complication of narcolepsy, which is also a risk in other sleep disorders, and that's having an accident due to fatigue. The urge to sleep is so sudden and irresistible that it's more like passing out than going to sleep. It's been known to happen while operating a motor vehicle or heavy machinery. Similarly, people who don't sleep well at night may be below par in the daytime, whether they feel an urge to sleep or not. Even minor fatigue can impair judgement, making vehicles and heavy machinery dangerous. Controlling the symptoms of narcolepsy can go a long way towards preventing accidents.