It is so funny that you asked about this today. We have had the same problem for the last month or so. It was really bad last night, the robin was hitting our bedroom window late into the night. It started again this morning at 5 am.
I looked online and found many recommendations. Several sites mentioned using tape or glass markers to make vertical lines on the outside of the window. One site mentioned using soap. I tried the soap since I had that available. I marked vertical lines on three different windows several hours ago. So far so good. It's actually raining right now and the soap is staying in place. Hopefully it will continue to work, we need a bit more sleep than robins do evidently.