zoloft tends to cause insomnia not provoke fatigue but you could be having an unusual reaction. It should be taken in the morning for this reason.
Severe nausea and fatigue that were not a problem before starting 6-mp could be signs that you are having a bad reaction to the 6-mp. Have you called your gi about your symptoms?
But going down on the prednisone can also make you feel really lousy - achey like you've got the flu, emotional, just plain bad. If you're waking up with a headache and your symptoms are worse in the morning you could also be experiencing adrenal insufficiency. Another thing to ask your gi about. You may need to slow your wean down depending on how fast you're going as a fast wean will make you feel worse.
Endocrinologist told us the rule she follows once a patient reaches 10 mg is to spit the dose between morning and evening and reduce the evening dose by roughly 1/2 each week (unless symptoms are a problem in which case wait 2 weeks to reduce dose) until finished then to spit the morning dose by 1/2 each week until done.
So to wean from 10 mg she had my son taking 5 mg in am and 5 mg in pm. Then 5 mg in am and 2.5 in pm. Then 5 mg in am and 2.5 every other day in pm (or 1.25 each day). Then he dropped to 5 mg in am and 1 mg pm (he felt lousy too) and stayed there for about 2 weeks. Then we dropped to 5 mg am and .5 mg pm then to just 5 mg in the am - etc. It took a long, long time to wean those last 10 mg.
Hope you feel better soon. Of course, you should ask your doctor before you change the way you take any of your meds, especially prednisone.