Norepinephrine (aka noradrenaline) is a neurotransmitter and a hormone that is involved in the regulation of a wide range of physiological functions, including heart rate, blood pressure, and the body’s response to stress. It plays a role in the regulation of attention, alertness, and motivation, and is also involved in regulating mood, sleep, and appetite. Imbalance in the levels of norepinephrine can contribute to, for example, depression, anxiety, attention deficit hyperactivity disorder (ADHD).