Medical News Today: Narcolepsy and insomnia share a chemical messenger called orexin in common. This peptide plays a role in activating parts of the brain that promote wakefulness.
People with narcolepsy may have fewer than normal nerve cells that produce orexin, while those with insomnia may have too much orexin or an inappropriate release of orexin at night. Too little orexin may cause sleepiness, and too much can cause an inability to sleep.
It is also possible that a person with narcolepsy may experience insomnia as one of their symptoms. In these instances, an individual with too little orexin may also experience insomnia.