“Neurotransmitters are chemical-like substances that travel between nerve cells across a synapse and determine whether a nerve signal keeps going or halts. Levels of two of these neurotransmitters, dopamine and serotonin, decrease during adolescence. The decrease in dopamine results in mood changes and problems with emotional control. The decrease in serotonin results in decreased impulse control. A third neurotransmitter, melatonin, increases in adolescence. Melatonin is responsible for circadian rhythms and the sleep–wake cycle. Its increase results in a need for greater sleep.”