Claritan,
head injuries can take a long time to heal. Work can be helpful emotionally, making you feel like you are functional even with lingering symptoms. Sometimes you just have to follow your instincts. If working a 40 hour week is draining you completely, cutting back a bit may help give you a bit more healing time. If work doesn't seem to be making things worse, and your Dr. has cleared you to go back, its probably ok.

The other thing is to consider what kind of work you are doing. If the symptoms you still have are likely to cause injury to yourself or your co-workers, (for instance poor coordination in a job that requires dextarity or using large equipment), then you need to re-think your decision to go back.

Hang in there.

Sam