In the wiki article for this film it says:
Did Nolan really write the final draft of Insomnia? It would make sense, and I wouldn't be surprised. I just thought he didn't write it. And if he did, why didn't he get credit? Someone help me out.To date, this is the only film that Christopher Nolan has directed without receiving at least a share of one of the writing credits, even though he wrote the final draft of the script.