Is tryptophan a sleep-inducing substance?

Herbs for sleep. In general yes. What they say about thanksgiving turkey is true. It is not a recommended otc herb because it can also be very harmful in high doses and years ago a tainted batch caused numerous cases of parkinson's disease. Hope this helps.
Hardly. Only in so far as it is a precursor to melatonin (and serotonin). It also carries the risk of causing eosinophilia-myalgia syndrome. And serotonin can cause insomnia. There is hardly enough tryptophan in turkey to cause sleepiness. Stick with melatonin if you want a natural sleep agent. Be aware that herbals are not regulated like rx meds and there is variability in the quality of what you get.