In my experience with 5 or 6 MRI's, there is a tension from the whole process. Dealing with the claustrophobic situation takes a lot of mental effort. The noise of the pounding is also a big stressor. To me it is like spending 5 or 6 hours in a crowded airplane. The constant noise, intense fine vibrations and limited movement makes it impossible to relax fully.

So, at least I understand how your body and brain are fatigued from the experience.