Many teens aren't getting enough bright light in the morning to reset their biological clocks, researchers have found.

Parents often complain their teens stay up late even though they are chronically overtired — the teenaged night owl syndrome, said Mariana Figueiro, program director at the Rensselaer Polytechnic Institute's Lighting Research Center in Troy, N.Y., and lead researcher of the study.

'It's very important not just to give light in the morning but also to remove light in the evening.'— Mariana Figueiro, researcher

Most teens miss out on the morning light because they rush off to school before the sun is up, leading to delayed sleepiness at night.

Too much light during the evening can have the same effect. Without a daily reset from exposure to blue light, sleepiness at night is delayed, the researchers said.

"You may put them in bed at 10 o'clock at night. They're going to be looking at the ceiling until their biological clock sends them a signal to fall asleep," Figueiro said.

In the study, published online this week in the journal Neuroendocrinology Letters, Figueiro and her colleagues found that 11 eighth graders who wore special glasses to prevent blue light from reaching their eyes experienced a 30-minute delay in falling asleep by the end of the five-day study.

It was the first field study to measure the effects in teens of getting less morning light.

"It's very important not just to give light in the morning but also to remove light in the evening," Figueiro said. "That combination is really a strong combination to help parents do something about the messy and the confused sleep cycles of teenagers."

Let in natural light

The school in the experiment was in Chapel Hill, N.C., and had large, south-facing windows to deliver daylight to most of the interior throughout the day.

Figueiro and her co-author found the onset of melatonin production in dim light was delayed in the teens studied. The hormone helps tell the body when it is nighttime, keeps the body on a regular sleep/wake cycle and prepares us for sleep, the researchers said.

During the experiment, participants were asked not to consume caffeine in case it interfered with the melatonin measurements taken from their saliva samples. Blinds were pulled down and all electrical lighting was kept off during sample collection. Light levels were also carefully measured.

To help teens stay alert during class, the study's authors suggest redesigning schools to allow in more natural light and having students go outside during daylight. Bright electric lighting may also be enough to activate the system, but the researchers did not test the idea.

Adding morning light and removing evening light are general tips, but parents also need to determine how their teenager responds to the light. Individual differences in when the core body temperature drops during the evening plays an important role in how light synchronizes the internal clock, Figueiro said.

Studies have shown that children who don't get enough sleep don't do as well in school and are more likely to have mood disorders such as depression. Lack of sleep has also been shown to contribute to weight gain, Type 2 diabetes and obesity.