To determine if kids are receiving their melatonin signals later, it helps to understand where they are in their sexual development. Many doctors make this determination using the Tanner Stages or Tanner Scale (sometimes called the Sexual Maturity Ratings). Developed by pediatric endocrinologist James Tanner, the scale measures the development level of three external sexual characteristics—genital development in boys, breast development in girls, and pubic and body hair distribution in both boys and girls—and provides five stages of development for each characteristic. Stage 1 marks the beginning of puberty for children and Stage 5 signals the attainment of adulthood. The further along in maturation kids are, the more likely they’ll be experiencing a delay in sleep onset and the later that delay may become. (If you haven’t seen your child streaking through the house lately, it’s likely that puberty has at least begun.)

Another way to measure melatonin levels and pinpoint the timing of the sleep cycle is to collect hourly saliva samples from your teen between noon and midnight in a sleep doctor’s office. If your teen keeps a careful sleep log for one or two weeks, though, it should provide all the information that’s needed to figure out how late the sleep delay is and where the sleep-onset clock is set.

Why do these brain changes occur? Why is melatonin secreted later? We don’t really know. But the brain is intricately complex, and the changes it experiences likely benefit the process of maturation in ways we have yet to appreciate. It’s also possible that evolution plays a role: Primitive cultures considered pubertal adolescents to be adults, and their ability to stay awake at night to stand watch may have made a valuable contribution to their societies.

For now we only know that in teens the timing of sleep is much later. We also know that the overwhelming majority of young people experience this Delayed Sleep Phase Syndrome, or DSPS, which puts them at odds with the world around them (especially their parents!). It also contributes to making them chronically drowsy and exhausted at times of the day that we think they should be at their peak.

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