I found this old information about getting fatigued and why.

Fatigue (Lassitude) Is a debilitating kind of overall weariness, which is unpredictable and out of proportion to the activity.

Any increase in your body temperature will temporarily worsen fatigue; conversely, air-conditioning or a cool drink will lower your temperature, enabling you to feel better and continue.

Some Causes Of Fatigue
*Nerve Fiber Fatigue (Conduction Failure)
*MS Process Fatigue (Due To Inflammation)
*Fatigue Of Handicap (Increased Effort)

*Fatigue from a current infection
*Iatrogenic Fatigue (Caused By Medication)
*Fatigue from disrupted Sleep
*Fatigue associated with Depression

Even a good night's sleep does not always relieve MS Fatigue. You wake exhausted, feeling like it is again time for bed:

It may take a few days of total rest, to recover from any over-doings. Fatigue is one of the most common, earliest, and troubling Multiple Sclerosis symptom.
Amen to that.