I haven't used Niacin (Vitamin B3) as a supplement for a sleeping agent but it can be used as such ... with caution.
Niacin, Tryptophan and Serotonin are all closely related. Your body requires 60mg of tryptophan to synthesise 1 mg of niacin. If your body is using an excess of tryptophan to produce niacin then serotonin levels will drop. If you diet is balanced though, then it swings the other way and serotinin levels via the existing tryptophan will go up.
The recommended doseage for Nacin is 15 to 20 mg per day (Adult males). Niacin is abundant in red meat, fish, poultry, nuts, and green leafy vegetables. Usually any foods high in protein will be sufficient to meet your daily niacin requirements without any need for supplementation.
If you are using Niacin as a 'supplement' then be aware that it can cause hystamine to be released which can cause a flushing (reddening) of the skin and be harmful to individuals with asthma. At high levels, Niacin can be toxic to the liver.
NP, anthrax.
You are correct on the non-flush variety being available now but that is the only thing which is lessened. The other health concerns, at higher consumption, still remain.
If you had a Niacin defficiency which produces a condition called pellagra then your prescribed dosage would be between 300 - 500mg per day (Adult).
Other than that and the fact that your diet is balanced there is no need to use it as a supplement.
Consider yourself lucky to have L-Tryptophan!
Does it "work"?
As for the cause of my insomnia.... I don't know
All I know is if I'm bored (shitty unmotivating job for weeks) I have some sleep problem (can't sleep at night and can hardly stay awake during the day)
You may just be fighting a losing battle against your internal clock. Some people are pre-programmed to be more alert at night than during daylight hours and visa-versa. If you are having great difficulties in sleeping and you find yourself bored and tired during the day then it might be an idea to change your routine completely. If possible, look for work which is more compatible with your internal clock. This can be difficult but it may help.
The tryptophan would also help you but, like you mentioned, it was banned as a supplement due to deaths resulting from a contaminated batch. I believe it is only available now on prescription.