For those that don’t have a regular schedule, I’ve been scheduling my deep work according to my work shifts. What are some problems with this?

Your deep work is for YOU and what you want to work on for yourself. In my opinion scheduling your deep work with your work shifts may pose an issue. Focusing on your job responsibilities and your own needs at the same time may cause you to neglect one for the other. Try carving out maybe 30 minutes to an hour a day and do your deep work during that time. Keeping on schedule will allow you to build good habits and with the consistency you’ll be able to make your deep work a lifestyle. You’ll enjoy it more and actually look forward to that time you’ve set aside for YOU.