There have been lots of studies on reiki and the understanding of how it works. Quantum physics and studies of the brain have shown the science of how reiki works. 

Reiki has been introduced to some hospitals and medical centres around the world, including here in the UK. Here are a few link to take a look at the amazing healing work.

https://lisapowers.co/reiki-the-unified-field/

https://www.collective-evolution.com/2016/03/11/how-reiki-actually-works-the-science-part/

https://www.cancerresearchuk.org/about-cancer/cancer-in-general/treatment/complementary-alternative-therapies/individual-therapies/reiki

https://www.reiki.org/reikinews/reiki_in_hospitals.html