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Poppy seeds have been shown to reduce cortisol levels in the body. Blend them in smoothies, bake with them or boil them and drink as tea. Be careful though, they make the body so relaxed sleep comes very easily. So they are good for insomniacs too.
Other plants that work against stress are adaptogens - rhodiola, moringa, pine pollen, ashwaganda, cacao, maca, holy basil, etc. Read up on adaptogens.