About 80% of people with multiple sclerosis (MS) have what the Centers for Disease Control and Prevention define as pathological fatigue—one that is not improved by bed rest and may be worsened by physical or mental activity, according to the National Multiple Sclerosis Society. In fact, studies have found that sleep disorders like narcolepsy, rapid eye movement (REM) sleep behavior disorder, and insomnia are commonly found among patients with MS.

But as a study in the Journal of Clinical Sleep Medicine points out, a significant number of sleep disorders remain undiagnosed in MS patients.

That’s why Robert S. Rosenberg, DO, FCCP, a board-certified sleep medicine physician and author of The Doctor’s Guide to Sleep Solutions for Stress & Anxiety, says it’s crucial for both patients and clinicians to check if sleep disorders like narcolepsy are also part of the underlying problem of fatigue-related issues. “If the patient is presenting with significant sleepiness, it is not uncommon for sleep apnea or narcolepsy to be overlooked,” Rosenberg says.

In fact, whether it’s due to a head injury, brain tumor, or an inherited disorder, MS is reportedly the fourth common cause of narcolepsy that is associated with other disorders.

There are two prevailing theories that point to why narcolepsy is more prevalent among MS patients, according to Rosenberg. One involves the common genetic links both MS and narcolepsy share; the other suggests that it’s due to the hypothalamic lesions that damage the hypocretin neurons.

“When [MS becomes a cause of narcolepsy,] we frequently see MS lesions in the hypothalamus,” says Rosenberg. “This is the area of the brain that is attacked in narcolepsy by what we believe to be an autoimmune process. In MS, the same areas are attacked by MS itself and we see the low levels of hypocretin, the wake promoting neuropeptide, in the cerebrospinal fluid similar to what we see in type 1 narcolepsy.”

While it may be tricky to untangle the differences between MS symptoms and symptoms from sleep disorders like narcolepsy marked by excessive daytime sleepiness, Rosenberg emphasizes that it’s crucial for sleep disorders to be on a clinician’s radar.

“When we have a patient with MS who is sleepy, we need to look for the underlying cause such as sleep apnea or narcolepsy,” Rosenberg says. “[I’m also interested in] a few anecdotal cases in the literature of patients with MS and narcolepsy, who have been treated with corticosteroids, with resolution of the narcolepsy.”