Johansson K, et al. - Researchers conducted this non-interventional cross-sectional study to evaluate fatigue and associated features in patients with multiple sclerosis (MS) or narcolepsy and healthy controls, as well as to determine whether clinical parameters distinguish fatigued and non-fatigued MS patients. Participants in the study were 34 MS patients, 15 narcolepsy type 1 patients and 17 healthy controls. Fatigued MS patients are more similar to narcolepsy patients than they are to non-fatigued MS patients, who are similar to healthy controls. Insomnia is the most common cause of fatigue in MS patients, with disease duration being the only clinical variable that distinguishes fatigued from non-fatigued patients. Variation in fatigue in fatigued patients cannot be explained by insomnia, daytime sleepiness, depression, or level of exercise.