Night terrors are a kind of sleep disorders, most often confused with nightmares. However, what distinguishes them is the fact, that night terrors happen during N-REM part of sleep, a while after falling asleep. The main characteristics are- not experiencing the imagery as it is possible in nightmares, sudden feeling of anxiety and worry while still being asleep and difficulty with awakening the sleeper. The person reacts negatively on physical contact and on the next day, in most cases, doesn’t recall what happened during the night. It is believed, that the issue of night terrors may be congenital. Although in many cases the causes are- medication that affects the central nervous system, sleep deprivation, stress and fever.
Onset insomnia is a sleep disorder, described as, and usually associated with the inability and difficulty to fall asleep and to continue sleeping. It has also been noted that this disorder may be caused by phobias in some cases. The main causes are- a sudden change in lifestyle, stress-inducing factors, several habitual factors such as consuming caffeine, or lack of any exercise. It is also common for people who suffer from anxiety to report trouble falling asleep. 
As opposed to onset insomnia, maintenance insomnia is connected to numerous interruptions during sleep and early awakenings. One of the most common causes for this type of sleep disorder is stress. Nevertheless in many cases, what holds the person from falling asleep up, is the worry that they will wake up tired. Thus, it keeps them more stressed and full of anxiety, which makes it impossible for them to fall asleep. Other causes are psychological disorders, medical issues such as chronic pain and substance use. 
Narcolepsy is described as one of the most dramatic and rare sleep disorders. It is a neurological disorder, that is linked to excessive daytime sleepiness. During the day the person may have the tendency to fall asleep for a short period of time, in many cases at inappropriate places. It also results in a fragmented sleep, meaning, that the sleeper suddenly awakens and than once again falls asleep to repeat the pattern. The characteristic factor is also the sleep paralysis that may occur after the sleeper awakens from dozing off. The exact cause of narcolepsy is not known. However it has been reported that there may be a connection to genetic disposition, which means, that just as in case of night terrors, this disorder may be congenital.
Narcolepsy connects with cataplexy, which is a muscle paralysis lasting usually a short period of time. In this case, the paralysis is a factor that should happen during the REM part of sleep, however, due to the nature of cataplexy, it happens while the sleeper is awake. Although this disorder is most often triggered by strong emotions, for example sadness, anger or laughter, it can occur at any time and place.