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leanguy I have suffered from insomnia for most of my life, and I too, have high histamine, although I have never had it definitely tested as it was quite obvious being that my body acts/acted like it was allergic to everything.
Interestingly, too many methyl donors makes me wired up too.
Unfortunately I have found that there are no really easy answers, and it has taken a little of this and a little of that to get my body to sleep well. And I still don't have the situation solved 100%.
I have found sleep to be this interplay between both environmental factors (ensuring low stress, lights off and dark in house 2 hours prior to bed, good sleep-habits i.e. same time, no reading in bed) & strategic supplements to address the underlying physiological causes.
For me a huge component was weak adrenals. My adrenals never tested in the abnormal range as per am/pm cortisol blood draws, but when I had saliva testing done my naturopath was able to identify that I had adrenal fatigue as my cortisol production didn't modulate accordingly to time of day & I had high night-time cortisol. I can provide you with more information if you feel that this might be an issue for you, in terms of what supplements and regimens worked for me to address. Related to the adrenal fatigue in my studies can be these issues with histamine. I have to eat a very, very restricted diet, and if I eat something as seemingly mundane as say a mango, I won't sleep either. As well, related to the adrenals was my problems with hypoglycemia. This accounts often for people's middle of the night wakings. It doesn't have to be an actual hypoglycemia low, but sugars falling too rapidly (relative hypoglycemia).
Lastly, it is evident that I have issues with melatonin production, and through the years I have on/off again used Tryptophan. However, *note my theory, I think some people especially with potential genetic mutations (snps) on MAO genes can have problems with that serotonin - melatonin pathway. I think I am one of those people. So if I take too much Tryptophan it is too stimulating, and too little does nothing.
Specifically with respect to the histamine - insomnia connection, for a long time I used antihistamines (over the counter) to sleep. Just make sure you don't get the non-drowsy kind. I often used gravol as well, which works as a anti-histamine...
Hope that helps...
S