We have not experienced this problem, but I do know that cortef (and other glucocorticoids) raise blood sugar levels.  Could it be that the large dose right before bedtime is keeping your child up because the increase in sugar is making them hyper? If your child needs an overall increase in meds. because of high levels, perhaps you can discuss with the ped. endo giving the higher dose in the daytime, so it doesn't interfere with the sleep issue.  I don't know that this will work, but perhaps it's worth a shot.  Good luck!