I think it's worth a try. It helped me with a painful muscle or tendon - I don't know what it was - on my leg (inner thigh). It had been sore for several years but was the least of my worries but then a few years ago got very painful such that putting on a pair of pants or lifting my leg to press the gas pedal would hurt noticeably. 2 weeks of red light therapy cut the pain in half and a couple more weeks it was gone altogether. Of course this was not an ME/CFS symptom but it still did something.
I have a sister with rheumatoid arthritis and she found that when she used my red light device in the morning, it cut her pain noticeably such that she didn't notice she'd forgotten her prednisone until late in the day. Ordinarily the pain would have started several hours earlier. She also noticed a boost in energy about an hour after using the device.
And one other sister had some type of nerve pain I believe in her hand or arm, and the red light cut that pain noticeably as well.
It didn't help me with PEM but, depending on your symptoms, might be worth a try. I first bought a large device from this company: mitoredlight.com - and it turned out to be too cumbersome, too heavy to maneuver for me. However, they now make stands which you can order separately which would solve this problem. I ended up getting a small device which is easy to move. However, depending on your level of functioning, I'd try the largest device you can afford or manage because it can cover a larger area of your body in one session.
mitoredlight,.com offers a 60-day money back guarantee and they honor it. I had to return the large device and ended up getting a small portable one. They did return 100% of the purchase price. They offer free shipping but I had to pay the return shipping, which I thought was fair.
It is calming and pets are said to like lying in front of it. My brother-in-law who is rather tightly wound and very skeptical by nature likes the way it makes him feel, and that's saying something! It might help with sleep too -
And more is not necessarily better - I'd read the instructions before using.