Premature babies, like the one being cared for by nursing extern Amanda Layne in Vanderbilt's neonatal intensive care unit, may benefit from exposure to a day/night lighting cycle.
Credit: Dana Johnson
NASHVILLE, Tenn. 22 August 2006: Keeping the lights on around the clock in neonatal intensive care units may interfere with the development of premature babies' biological clocks.
That is the suggestion of a new study reported in the August 21 issue of the journal Paediatric Research.
The study, which was headed by Douglas McMahon, professor of biological sciences at Vanderbilt University and an investigator at the Vanderbilt Kennedy Centre for Research on Human Development, reports that exposing baby mice to constant light keeps the master biological clock in their brains from developing properly and this can have a lasting effect on their behaviour.
"We are interested in the effects of light on biological clocks because they regulate our physiology extensively and also have an important effect on our mood," McMahon said. "This study suggests that cycling the lights in NICUs may be better than constant lighting for premature babies' from the perspective of developing their internal clocks."
Every year about 14 million low-weight babies are born worldwide and are exposed to artificial lighting in hospitals.
"Today, we realise that lighting is very important in nursing facilities, but our understanding of light's effects on patients and staff is still very rudimentary," said William F. Walsh, chief of nurseries at Vanderbilt's Monroe Carrel Jr. Children's Hospital. "We need to know more. That is why studies like this are very important."
Although older facilities still use round-the-clock lighting, modern NICUs, like that at Vanderbilt, cycle their lighting in a day/night cycle and keep lighting levels as low as possible, Walsh said. Covers are also kept over the isolets that hold the babies in an effort to duplicate the dark conditions of the womb.
The finding that exposure to constant light disrupts the developing biological clock in baby mice provides an underlying mechanism that helps explain the results of several previous clinical studies.
One found that infants from neonatal units with cyclic lighting tend to begin sleeping through the night more quickly than those from units with constant lighting. Other studies have found that infants placed in units that maintain a day/night cycle gain weight faster than those in units with constant light.
The research is a follow-up from a study that the McMahon group published last year which found that long periods of constant light disrupt the synchronization of the biological clock in adult mice.
In all mammals, including mice and humans, the master biological clock is located in an area of the brain called the suprachiasmatic nuclei (SCN). It influences the activity of a surprising number of organs, including the brain, heart, liver and lungs and regulates the daily activity cycles known as circadian rhythms.
The SCN is filled with special neurons that are wired in such a way that their activity varies on a regular cycle of roughly 24 hours. In a normal brain, the activity of these clock neurons is synchronized to a single cycle which is set by the 24-hour day/night cycle.
McMahon's previous study found that the SCN neurons in adult mice begin drifting out of phase after a mouse is exposed to constant light for about five months and that this is accompanied by a breakdown in their ability to maintain their normal nocturnal cycle.
"After we got this result, my post-doctoral fellow, Hidenobu Ohta, who is now a pediatrician at Tohoku University Hospital in Japan, wanted to study the impact of constant light on newborn mice because he was interested in finding out whether the use of constant light in NICUs may be having a similar effect," McMahon said.
Newborn mice provide a good model for premature human infants because baby mice are born at an earlier stage of development than humans, a stage closely equivalent to that of premature babies.
"We found that the newborn mice were even more vulnerable to the effects of constant light than the adults," McMahon said.
