You may be lacking in Tryptophan so without supplementation, your serotonin could be low allowing anxiety to creep in. Tryptophan is a precursor to 5-HTP then serotonin then another step and then melatonin. If you are lacking in melatonin, you could also be lacking in serotonin (anxiety).

5-HTP is often easier to find than L-Tryptophan.

So, the L-Tryptophan reduces the anxiety and its absence allows anxiety to increase.