Anything that stimulates your para-sympathetic nervous system, such as more relaxation, specific breathwork techniques, massage etc. You may also have to invest in some (tech) devices like better mattress, smart watch for tracking and bio feedback etc. There are some useful hacks too: the temperature in your bedroom, or a cold shower before bed, specific herbs, magnesium etc.

One of my favourite persons on breathwork, Patrick McKeown, has recently published a book on the subject. I haven't read it, but it would be worth looking it up.

The issue with your arm is probably just a symptom. Investigate the root cause. It may take time, but put your mind to the task. A 15 min visit at the doctor's office isn't enough, and this is probably the reason why he asked you to focus on improving your sleep, because better sleep quality usually fixes most health issues... IMO, he gave you the best advice.

I'm also working on my sleep, and find this topic fascinating.

Good luck to you.