First, a reminder of the basic biology: After puberty, adolescents are no longer the morning larks of their younger years. They become rewired as night owls, staying awake later and then sleeping in. This is not part of a feckless project to frustrate parents, but is driven by changes in the way the brain responds to light.

New technology habits aren’t helping. More teenagers now turn to activities involving screens at night. According to a report this year from the Pew Research Center, some 95 percent of children aged 13 to 17 now have access to a smartphone, up from 37 percent in 2012 and 73 percent in 2015. A Centers for Disease Control and Prevention survey from 2017 reveals that 43 percent of high-school students are playing computer or video games for more than three hours on an average school night. Given the binge viewing encouraged by the likes of Netflix and YouTube and the pressure to nurture social networks like Facebook, Instagram and Snapchat, the total screen time for youngsters is probably well in excess of six hours a day, on average.

The growth in screen time is particularly problematic for sleep. Not only does it eat into the time available for rest, but the blue light emitted by LEDs, TVs, tablets and smartphones suppresses the body’s secretion of melatonin, the hormone that signals it’s time to sleep. Overdosing on screens at night effectively tells the brain it’s still daytime, delaying the body’s cues to sleep even further.

Parents should set real limits on screen time, model responsible use of devices and praise children who show signs of regulating their own media consumption. In the hour before bedtime, there should be a moratorium on bright lights in the home, avoiding devices and harsh LED bulbs often found in kitchens and bathrooms.

Excessive screen use is compounded by a dangerous tradition: starting high school abnormally early. Based on data available from 2015, 86 percent of high schools started before 8:30 a.m., and one in 10 high schools had a start time before 7:30 a.m. Prying a teenager out of bed at 6 a.m. to get to school is the equivalent of waking an adult at 4 a.m. The brain will be at its least active in the 24-hour cycle, which explains the monosyllabic grunts of teenagers as they lumber to the school bus.