Medical: Natural light can disturb teen's sleep patterns

If your teenage cub turns into a bear each spring with the onset of longer days, researchers say that, as with many things adolescent, blame a hormone.

In this case, it's melatonin, a hormone that signals the body of the onset of darkness, a gradual buildup that usually precedes falling asleep by several hours.

Researchers at the Rennselaer Polytechnic Institute's Lighting Research Center in Troy, N.Y., measured light exposure and melatonin levels in a group of eighth-graders at a nearby middle school during winter and spring test periods.

They found during the spring test that students started melatonin buildup an average of 20 minutes later than during the winter. Based on sleep logs kept by the students, they were falling asleep an average of 16 minutes later and sleeping an average of 15 minutes less during the night compared to the winter.

Mariana Figueiro, an associate professor who conducted the study with center director Mark Rea, said the delay in sleep onset may lead to teen sleep deprivation and mood changes, increased risk for obesity and perhaps underperformance in school.

"This is a double-barreled problem for teenagers and their parents," Figueiro said. "In addition to the exposure to more evening daylight, many teens also contend with not getting enough morning light to simulate the body's biological system, which also delays teens' bedtimes."

The study, which had the students wear a small head-mounted device that measures natural light, demonstrated that it was exposure to extended daylight hours due to the seasonal change, rather than evening electronic light at home, that had the biggest effect on delayed sleeping patterns.

Figueiro said the research, published in the July issue of Chronobiology International, supports the theory that the entire 24-hour pattern of light and dark exposure influences synchronization of the internal biological clock with the solar day.

"As a general rule, teenagers should try to increase their morning daylight exposure year-round and decrease evening daylight exposure in the spring to help ensure they will get sufficient sleep before going to school," she said.

Among adults seeking to catch up from a sleep deficit, a study published in the August issue of the journal Sleep suggests that sacking out for up to 10 hours on a weekend is helpful, but not necessarily enough to cure all the negative effects of sleep deprivation.

Researchers at the University of Pennsylvania School of Medicine in Philadelphia set up a sleep-restriction test in a group of 159 adults, each lasting five days, followed by various doses of recovery sleep (0 to 10 hours). All the subjects got 10 hours in bed for each of two nights before the tests started and one control group got that much opportunity to sleep on all nights.

Various assessments of alertness and reaction time along with measures of sleepiness were taken each morning. The results showed that impairments accumulated over the five days among subjects who got less than four hours of sleep a night.

For the most part, the recovery of alertness was linked to the amount of makeup sleep, said David Dinges, lead investigator for the study and chief of the school's sleep and chronobiology division.

"However, the sleep restriction was severe enough that recovery of alertness was not complete following a single night of extended sleep, indicating a sleep debt remained," the researcher said.

Dinges and his colleagues noted that the body clock's limits of sleep duration may prevent an individual from getting enough sleep in one night to recover from an extended period of little or no sleep, something many adults experience when they attempt to sleep in all day on a weekend.

Reach Lee Bowman at bowmanl@shns.com.