All living things have circadian ( or body) clocks which are biochemical mechanisms that allow them to organise their sleep and waking across the 24-hour cycle of a day. It’s been known for decades how light adjusts this rhythm through the release of Melatonin, our sleep hormone. Now using fruit flies, Scientists at the University of Michigan have discovered that we have nerve cells which detect changes in environmental temperature that are also involved in sleep regulation. Fruit flies in particular were selected as they have nerves that govern their circadian clocks which are very similar to those found in humans.
To study how the fruit fly neurons responded to external temperature, Swathi Swathi Yadlapalli and Chang Jiang developed an optical imaging and temperature control system that enabled them to take a snapshot of nerve activity in the circadian clock network of fruit flies when the flies are exposed to heat or cold stimulus.
Their research showed that circadian clock nerve cells use thermoreceptors to constantly monitor the temperature of their environment, with colder temperatures exciting sleep promoting nerves. They found that even mild changes in temperature have physiological effects on clock nerves that control sleep timing. Finding this in fruit flies suggest that the same nerves could provide a similar role in the humans.
Orie Shafer, principal investigator of the study commented that rather than our body temperature being a steady 98.6 degrees, it fluctuates through the day. “The circadian system produces a daily rhythm in temperature which is an important cue for when it’s time to go to sleep.”
As we get closer to bedtime our circadian clocks cool our internal body temperature, and as we move towards waking up these body clocks turn up the heat. This is regardless of the temperature of the room we’re sleeping in. But showing that circadian clock neurons in fruit flies use external temperature to trigger sleep suggests that some clock neurons in humans could be similarly sensitive.
“It looks like clock neurons are able to get the temperature information from external thermoreceptors, and that information is being used to time sleep in the fly in a way that’s fundamentally the same as it is in humans,” Shafer said. “As temperature drops, these neurons that promote sleep become excited, and that really entrains the sleep activity cycle to external temperature cycles. It’s precisely what happens to sleep in mammals when internal temperature drops.”