Reasonable accommodations usually have to allow you to perform the essential functions of your job.
One of the problems with a request for reduced work hours, if you are full-time, is that by definition you are not performing all of the duties of the job that you were hired to do. Very few people can perform a full-time position on a part-time basis. When these issues get litigated, courts usually do not think that it is reasonable for force the employer to incur the costs of hiring someone on a part-time basis to do the work that you can't do because of your reduced work hours. If your schedule is unpredictable, this solution often is even more likely to be deemed "unreasonable".
You have not stated what your job requires of you nor have you stated what your condition is. I am not suggesting that you post that information here. However, you need to think about how a reasonable accommodation will allow you to perform the essential funcitons of your job. You also must realize that the courts will not impose an "unreasonable" burden on the employer.
In othe words, you are not entitled to part-time hours solely because that is what your doctor recommends as an accommodation.