Another interesting article popped out of the AI selected articles:
Vitamin D deficiency changes the intestinal microbiome reducing B vitamin production in the gut. The resulting lack of pantothenic acid adversely affects the immune system, producing a “pro-inflammatory” state associated with atherosclerosis and autoimmunity. 
Vitamin D blood levels of 60-80 ng/ml promote normal sleep. The present study was undertaken to explore why this beneficial effect waned after 2 years as arthritic pain increased. Pantothenic acid becomes coenzyme A, a cofactor necessary for cortisol and acetylcholine production. 1950s experiments suggested a connection between pantothenic acid deficiency, autoimmune arthritis and insomnia. The B vitamins have been shown to have an intestinal bacterial source and a food source, suggesting that the normal intestinal microbiome may have always been the primary source of B vitamins. Review of the scientific literature shows that pantothenic acid does not have a natural food source, it is supplied by the normal intestinal bacteria.
Three months of vitamin D plus B100 resulted in improved sleep, reduced pain and unexpected resolution of bowel symptoms. These results suggest that the combination of vitamin D plus B100 creates an intestinal environment that favors the return of the four specific species, Actinobacteria, Bacteroidetes, Firmicutes and Proteobacteria that make up the normal human microbiome.
This lead to some related articles (also old)
It was found that the time after which changes in vitamin levels developed as well as the intensity of changes differ, depending on the experimental model used, but the sequence of vitamin disappearance from the organism is always the same. The first to disappear were vitamins B1, followed by vitamin B6 and pantothenic acid.Changes in the levels of certain vitamins from B group under conditions of vitamin deficiency. 1976
- Decreased serum vitamin B12 and vitamin D levels affect sleep quality in children with familial Mediterranean fever.
From the above we see a cascade starting with deficient Vitamin D
- Reduced Vitamin D
- Reduced Thiamine (B1)
- Reduced pyridoxine (B6)
- Reduced Pantothenic Acid (B5)
- eventually reduced Cobalamin (B12)
- ” affect VB12 level, indirectly, by reducing 25(OH)D level in middle aged women. ” 
For CFS specifically we have Ana Dorothea Hoeck’s research