Birth of the light.

The brain's circadian pacemaker, which regulates daily biological rhythms, responds sensitively to light almost immediately after birth, at least in baboons, a new investigation finds. This result raises the possibility that the bright artificial lighting typical of hospital nurseries sometimes wreaks havoc with the biological rhythms of human babies, according to neurobiologist Scott A. Rivkees of Yale University School of Medicine and his colleagues. Baboons display strong daily rhythms in their behavior and hormonal secretions, making them a good model for human circadian activity, the researchers state in the Jan. 7 Proceedings of the National Academy of Sciences.

Rivkees' group placed five newborn baboons in incubators and maintained them on regular 12-hour cycles of light and darkness for 2 days. Three animals who were then exposed to 45 minutes of bright light at night exhibited increased activity in the suprachiasmatic nucleus, a brain structure that generates circadian rhythms; they also displayed elevated concentrations of a protein manufactured by the suprachiasmatic nucleus.

Another four newborn baboons experienced either alternating light and dark periods or total darkness for several days after birth. During that time, the infants developed different cycles of activity and rest. Exposure to constant dim light then yielded large shifts in activity-rest cycles, indicating a resetting of the infants' biological clocks.