While you sleep, your brain’s circadian clock may be constantly monitoring changes in external temperature and incorporating that information into the neural network that controls sleep. These were the findings suggested in a study published in Nature and supported by the National Institute of Neurological Disorders and Stroke (NINDS), a branch of the NIH.
The study was done in flies, whose evolutionarily conserved circadian clock networks appear to be essentially the same as those in humans. The researchers used a fluorescent protein that changes from green to red when neurons fire in order to observe activation of different parts of the circadian clock in response to temperature change. An area of the fly brain’s circadian clock called DN1p was observed to increase in activity when cooled and become less active when heated.
"We knew that light excites the circadian clock overall and that light and heat typically increase at the same time, so it was completely unexpected to find a region of the clock that increased its activity in response to cooling," said Orie Shafer, Ph.D., associate professor at the University of Michigan, Ann Arbor, and senior author of this study.
To explore whether DN1p is responsible for resetting the clock to a new heating/cooling cycle, the researchers blocked the activity of DN1p neurons or eliminated them genetically. In both cases, the flies’ ability to retrain their sleep following changes in temperature was affected.
The researchers next wanted to determine whether the temperature changes are sensed directly or via nerve impulses from sensory organs in the body. When the sensory organs were knocked out through genetic manipulation or physical removal, DN1p neurons no longer responded to temperature changes. This indicates temperature is sensed in the body and that information is fed to the internal clock.
Image: This image shows an increase in 'hotter' colors after cooling indicates more firing in neurons of the circadian clock. Image courtesy of the Shafer lab, University of Michigan, Ann Arbor.