Extended hours of daylight in the spring lead to later bedtimes for teens, causing them to be sleepier in the morning, researchers reported Tuesday. The conclusion might seem obvious, but the researchers from the Lighting Research Center at Rensselaer Polytechnic Institute in Troy, N.Y., have provided a biological basis for the finding. They conclude that delays in melatonin production by the teens' bodies leads to the later bedtimes.
Melatonin is the naturally occurring hormone that regulates sleep on a 24-hour cycle. Normally, melatonin levels start rising two to three hours before the onset of sleepiness. Sunlight interferes with melatonin production, however, and can disrupt that cycle. The same team reported earlier this year, for example, that preventing exposure of teens to bright lights in the morning
could cause a 30-minute delay in sleep onset, leading to excessive daytime sleepiness.
Sleep researchers Mariana G. Figueiro and Mark Rea, director of the lighting center, studied 16 eighth-grade students at Algonquin Middle School in upstate New York, fitting them with a small head-mounted device that measured exposure to sunlight as well as rest and activity patterns. They also measured blood levels of melatonin in the students. They reported in the journal Chronobiology International
that the youth experienced a 20-minute delay in the onset of melatonin on one day spring compared with a day in winter. Sleep logs kept by the students indicated that the delay in melatonin onset was accompanied by a 16-minute delay in sleep onset and a 15-minute reduction in sleep duration.
Over time, when coupled with having to rise early for school, this delay in sleep onset may lead to sleep-deprivation and mood changes and increase the risk of obesity, Figueiro said.
-- Thomas H. Maugh II