Why not ask if you can have a graded return to work plan where you start off working less than 40 hours a week? I came back on 20 hours, if I'd gone straight back to full time I think I would have failed, which neither you nor your employer would want.

Even if you can manage 40 hours, it could leave you with no life outside work if you're putting every ounce of what mental energy you have into your work hours then collapsing when you get home. This would be stressful and depressing.

Fatigue is a well known symptom of head injury so they should understand, my employer did. Just a thought.