Constant exposure to artificial hospital lighting may damage the development of premature babies' biological clocks, research suggests.
Lighting may be key in neonatal units
Tests showed exposing baby mice to constant light keeps the master biological clock in their brains from developing properly.
Researchers said this could contribute to an increased risk of mood disorders, such as depression.
The Vanderbilt University study appears in the journal Pediatric Research.
The researchers say their findings suggest special care baby units should try to minimise a baby's exposure to artificial lighting - possibly by using a day/night cycle.
Each year about 14 million premature babies are born worldwide, and many are exposed to artificial lighting in hospitals.
Previous research has found infants from neonatal units with cyclic lighting tend to begin sleeping through the night more quickly, and gain weight faster than those from units with constant lighting.
In all mammals the master biological clock is located in an area of the brain called the suprachiasmatic nuclei (SCN).
It influences the activity of many organs, including the brain, heart, liver and lungs and regulates the daily activity cycles known as circadian rhythms.
The SCN is filled with special clock neuron cells whose activity is synchronized follow the 24-hour day/night cycle.
The Vanderbilt team had already shown SCN neurons in adult mice begin drifting out of a phase after the animals were exposed to constant light for about five months.
This is accompanied by a breakdown in their ability to maintain their normal nocturnal cycle.
The latest study found that newborn mice were even more vulnerable to the effects of constant light than the adults.
The Vanderbilt team used genetically modified mice whose clock neurons produced a bright glow when active.
They found neurons in baby mice exposed to the normal light cycle quickly became synchronised.
In contrast, neurons in those animals exposed to constant light were unable to maintain coherent rhythms.
However, when these animals were then exposed to the day/night cycle of light their neurons rapidly fell into line.
The scientists then exposed some mice to constant light for a much longer period - and found that two-thirds were unable to establish a regular pattern of activity on an exercise wheel.
Conversely, newborn mice who spent their first three weeks in a day/night cycle were able to maintain their normal daily rhythm when later exposed to constant light.
Lead researcher Dr Douglas McMahon said more work was needed to establish whether disruption of a baby's biological clock could increase their vulnerability to mood disorders.
"All this is speculative at this point. But, certainly the data would indicate that human infants benefit from the synchronizing effect of a normal light cycle."
Professor Andrew Shennan, an expert in obstetrics for Tommy's, the baby charity, said the link between light exposure and its effects on mood and behaviour were quite firmly established.
"Currently, any babies who are admitted to a special care baby unit are going to be exposed to incredibly harsh lighting to facilitate care, at anytime day or night that it is needed.
"Many units now try and reduce adverse stimuli including lighting for periods during the day and at night.
"As a result of this research the potential benefit of reducing unnecessary light exposure must now be investigated, as it would seem that there is a strong possibility that this could improve the development of the body clock."
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.