Normally in our bodies, l-tryptophan + B6 are involved in making Serotonin. MethylB12 is then involved with serotonin to make melatonin. So if you are having sleeping issues, taking some melatonin may help until your system starts making it again. Low B12 often results in poor sleeping. And that takes a while to fix itself too.

The SSRI drugs focus the serotonin on the synapse, and hence the cell eventually stops making more, and when the drug is removed suddenly, the cells have none in back up function.

Taking some tryptophan is often helpful to start things back up.
As I said it is more gentle than using 5-HTP which is faster and more like a drug. Taking the tryptophan away from meals will allow it to pass thru the blood brain barrier more effectively, as there is a competition with other amino acids there.

You don't even need it every day... starting at twice a week may slowly jumpstart your own synthesis.