Natalizumab has been shown to positively impact fatigue and cognition. The mechanism by which this occurs is unknown. Change in quality of sleep is one possible etiology of these findings. This study will help to elucidate the mechanisms that lead to reduced fatigue/sleepiness and less cognitive dysfunction in MS patients who go on Natalizumab therapy. Understanding these factors may help neurologists better differentiate between the different therapeutic options for MS and how they may impact symptoms that negatively affect quality of life.