Surgeons usually prefer to start operating before 8AM. They are awake, the OR is clean, the staff is fresh, and they have all day afterward to monitor for complications (and jet out early if everything goes well). The worst surgery times are probably starting around 7PM and going until 6AM. Those are usually emergencies, the OR has been in use all day, the surgeon probably has less experience as they are earlier in their career, the preferred materiel might be waiting to be autoclaved, etc.