Do you have anxiety and high adrenaline? are you sympathetic or parasympathetic dominant?
I tried L-theanine but it gave me side effects, it kept me awake the whole night and gave the weird feeling of being really sleepy and spacey but agitated at the same time.
But it might be just me cause all the supplements that are supposed to be calming, become stimulating in my case.
So I say give it a try if you too have high adrenaline and feel the need to relax, it works for many people.
Magnesium is also calming. It won't lower cortisol if it's already low, only if it's high, it can also help with sleep.