I understand your concern and situation. First thing in my opinion this is not appropriate to have excuses from doctor for your problem. You should frankly tell your employer the problems which you are facing due to this and tell them to change your shifts. There must be other employees in your office which are also working in shifts with you. They were also having the similar issues like you. My advice is you all should try to develop the good working atmosphere at your office with the help of employees and employer. The anxiety and insomnia is due to stress, you should visit a psychiatrist and talk to him. Accordingly he will plan a management for you. Once these stress factor will be taken care of ulcerative colitis will also reduce. Hope this will help you.