March 12, 2018Circadian Rhythms
The circadian clock is a fundamental process found in nearly every living organism that coordinates sleep behavior with changes in the environment. The link between the light/dark cycle and the onset of sleep is well recognized; however, changes in temperature also appear to affect sleep patterns in humans.
According to a study published in Nature (21 February 2018), circuits in the brain act as an internal clock to tell us it is time to sleep and to control how long we then stay asleep. The new study in flies suggests a part of that clock constantly monitors changes in external temperature and integrates that information into the neural network controlling sleep. By using a special fluorescent protein that changes from green to red when neurons fire, the authors watched the activity of different parts of the fly brain's circadian clock while they increased or decreased the surrounding temperature. To the surprise of the authors, an area in the fly brain's circadian clock called the DN1p increased its activity when cooled and became less active when heated.
As we all know when travelling, the circadian clock can be reset over time in response to new day/light cycles. Since the clocks of flies can be retrained to new cycles of either light or temperature, the authors next looked at whether the DN1p is involved in resetting the clock to a new heating/cooling cycle. Because DN1p neurons are thought to be sleep-promoting, the authors blocked their activity or eliminated them genetically. Both affected the flies' ability to retrain their sleep cycle in response to changes in temperature, highlighting the importance of the DN1p for the control of sleep behavior. According to the authors, because flies' bodies are translucent and their clock neurons can respond to light directly, the next question asked was whether temperature worked in the same way or required external organs.
In flies, temperature could be sensed directly by neurons in the brain or via nerve impulses from sensory organs in the body. To distinguish between the two, the authors genetically manipulated or physically removed the sensory organs and found that the DN1p neurons no longer responded to changes in temperature. This meant that the clock interprets temperature signals from the body rather than sensing temperature changes directly.
The circadian clock of larger animals and humans is also sensitive to changes in temperature, and because of their larger size, would require input from external sensory organs. The fact that, despite its small size, the fly clock also relies on temperature sensors outside the brain suggests that the findings of this study could have broad implications in the control of sleep in humans.