in the morning, teens’ brains signal both sleepiness and wakefulness at much later times.

Until recently, scientists believed that the human brain was nearly fully developed by the time its owner reached the age of 3. Babies are born with most of the brain neurons they’ll ever have, and unnecessary cells are weeded out during the last several months of gestation. By age 3, it was thought, the brain was pretty much a finished, polished product.

Not so, we now know—the brain continues developing well into the 20s. Dr. Jay Giedd, a neuroscientist at the National Institutes of Health, has led a number of studies that show that the brain undergoes enormous change around the time of puberty, a thinning or “pruning” of neurons, or nerve cells, that doesn’t stop until about age 25. Giedd has found that other changes, from a speeding up of neural transmissions to growth in several key areas of the cerebral cortex, occur in the brain as well. From these and others studies, it’s clear that the teenage brain is still very much a work in progress.

Part of this ongoing brain development is evidenced in the adolescent tendency to fall asleep and wake up later than other folks. Chemicals in the brain, called neurotransmitters, send and receive messages, some of which signal when it’s time to go to sleep and others when it’s time to wake up. For example, norepinephrine, dopamine, serotonin, and hypocretin promote alertness and keep the brain awake. Cholinergic transmission is involved in wakefulness and rapid eye movement