Assuming you have sufficient mB12 methylfolate will be converted to THF. THF is used to covert histidine to glutamate (instead of histamine). This can cause insomnia for some people. There are a number of things that can help with glutamate excitotoxicity:
1. B6: Too much causes issues for some of us, but if you aren't taking any, it is involved in interconverting glutamate to other things.
2. L-Carnitine: Increases glutamate affinity for metabotropic glutamate receptors. These receptors increase our appetite and generally pull glutamate out of circulation and back into storage.
3. Vitamin K (MK-4): Gets calcium out of the system. Calcium is a second-messenger for NMDA receptors (the problematic type of glutamate receptor).
4. Manganese: Necessary to convert glutamate to glutamine. Which seems to help some people. Unless you are one of the people that it gives horrible horrible insomnia to. MK-4 deficiency might be the deciding factor, but I am far from sure.
5. Magnesium: Inhibits glutamate receptors.
6. Boron or if you can't do that Molybdenum: Mo does and Boron seems to help with/be necessary for the activity of SUOX, which converts sulfites to sulfates. Sulfites combine with cystine to form a glutamate analog, so reducing sulfites numbers reduces glutamate excitotoxicity. On the other hand, this seems to be a case where the body adapts to having the glutamate analog around, so after a while Boron and Molybdenum no longer made me tired or helped with sleep--although they still help with sulfites, so I still take one of them. For myself taking both of them causes insomnia, so I would suggest you choose one.
7. Lithium: Yasko claims (and seeing as this is purely a clinical insight, I might believe her) that higher doses of B12 depletes lithium in some people. I'm not sure exactly how Lithium helps, although I see speculation that it antagonizes NMDA receptors in a similar fashion to magnesium.