Noradrenaline
A close cousin of adrenaline, also called norepinephrine, that gears you up. In the brain it raises alertness and attention; released by the body's alarm nerves, it speeds the heart and raises blood pressure.
Noradrenaline is a chemical messenger that works both in the brain and in the body. In America it's called norepinephrine; same thing. Evidence: solid Most of the brain's supply comes from one tiny cluster of cells in the brainstem, the locus coeruleus, which sends it almost everywhere and helps set how alert and attentive you are. Evidence: solid The body's alarm nerves, the sympathetic system, release it too: it speeds the heart and narrows blood vessels, which raises blood pressure. Evidence: solid What spills over from those nerves into the blood is most of what a blood test picks up; the adrenal glands add a little. Evidence: solid
It also helps charged moments stick. In animals, noradrenaline working through the amygdala after an arousing event strengthens the memory of it. Evidence: decent In people, propranolol, a drug that blocks its beta receptors, weakened memory for an upsetting story but not for a matching neutral one. Evidence: decent
Where it gets oversold: the cold plunge. In a small group of young men, an hour in 14°C water raised the noradrenaline in their blood by 530%. Evidence: decent That was measured in the blood, after an hour. It says nothing direct about your brain after a three-minute dip. Evidence: decent
Why it matters
Several hacks use it as a middle step. Coffee after learning may work partly by letting it run freer; the study's authors proposed that, they didn't measure it. Evidence: worth trying In the study behind hard exercise after learning, six minutes of hard cycling raised a saliva marker of it in older adults, who then remembered more of the pictures they'd just seen. Evidence: decent The jolt from the cold plunge runs through the alarm nerves. Evidence: decent The system that settles you afterwards is the brake. Evidence: solid