July 26, 2010
Science Daily/Rensselaer Polytechnic Institute
In the spring, later sunset and extended daylight exposure delay bedtimes in teenagers, according to new research.
"Biologically, this increased exposure to early evening light in the spring delays the onset of nocturnal melatonin, a hormone that indicates to the body when it's nighttime," explains Mariana Figueiro, Ph.D., associate professor. "This extended exposure adds to the difficulties teens have falling asleep at a reasonable hour."
Over time when coupled with having to rise early for school, this delay in sleep onset may lead to teen sleep deprivation and mood changes, and increase risk of obesity and perhaps under-performance in school, according to Figueiro.
"This is a double-barreled problem for teenagers and their parents," says Figueiro. "In addition to the exposure to more evening daylight, many teens also contend with not getting enough morning light to stimulate the body's biological system, also delaying teens' bedtimes."
Measuring "Circadian Light"
In the study, the Algonquin Middle School students were exposed to significantly more "circadian light" in the early evening during spring than in winter, resulting in both delayed melatonin onset and shorter self-reported sleep durations. Each subject wore a Daysimeter, a small, head-mounted device developed by the LRC to measure an individual's exposure to daily "circadian light," as well as rest and activity patterns. The definition of circadian light is based upon the potential for light to suppress melatonin synthesis at night, as opposed to measuring light in terms of how it stimulates the visual system.
This study, sponsored by the U.S. Green Building Council (USGBC) and, in part, by a grant from a Trans-National Institutes of Health Genes, Environment and Health Initiative (NIH-GEI), is the first to relate field measurements of circadian light exposures to a well-established circadian marker (the rise in evening melatonin levels) during two seasons of the year.
In a previous field study, also funded by USGBC and NIH-GEI and published in Neuroendocrinology Letters, Figueiro and Rea examined the impact of morning light on teen sleep habits and found that removing short-wavelength (blue) morning light resulted in a 30-minute delay in sleep onset by the end of a five-day period.