Does this imply that even if one take L-tryptophan at daytime, it will not be converted to Melatonin before night, or does the above imply that the L-tryptophan is taken before going to bed?
The process is regulated pretty tightly, and it can't be dependent on trp levels alone, or we wouldn't be able to sleep when we're hungry or otherwise in a state of low free trp. I don't know if this is the particular pathway that is regulated by light levels, but that's one example of how there can be multiple regulatory inputs. The sleep / wake cycle is too important to be weakly regulated.
I just recently was experimenting with 500mg of trp an hour before bed. It definitely works for me. After taking it for about a week, I noticed that the effects were not as strong, at least in the sleep induction phase. I'm noticing that I sleep pretty well without it, and have a sense that I'm sleeping better than before I started taking, even though I stopped recently. Could that be a long-lasting increase in my free trp pool?