"Brain thermometer" circuitry might explain those summer afternoon naps
The relationship between temperature and sleep is a highly complicated one, but researchers continue to tease out valuable new insights around how the heat and cold can influence our rest. A new study on fruit flies may help explain why siestas came to be such an important habit in many cultures, revealing a type of brain thermometer that kicks sleep-promoting cells into action as the mercury starts to climb.
The research was led by neurobiologists at Northwestern University who set out to explore the biological mechanisms behind the connection between temperature and our sleep-wake cycles. Their experiments centered on a popular model for studies on human physiology, the humble fruit fly, which shares many of the same genes as us.
Previously, the researchers had discovered a type of sensory map in the brains of fruit flies that enables them to detect and respond to different temperatures and humidity levels, helping them avoid dangerously hot conditions. To build on this, the scientists conducted deeper analysis of fruit fly neurons, and the ways their behavior is influenced by shifts in temperature.
“Changes in temperature have a strong effect on behavior in both humans and animals, and offer animals a cue that is time to adapt to the changing seasons,” said study author Marco Gallio. “The effect of temperature on sleep can be quite extreme, with some animals deciding to sleep off an entire season – think of a hibernating bear – but the specific brain circuits that mediate the interaction between temperature and sleep centers remain largely unmapped.”
This work led to the identification of what are described as "absolute heat" receptors in the heads of fruit flies. These "brain thermometers" respond to rising temperatures by activating a class of neurons related to the sleep-wake cycle. This mechanism was found to kick into gear above the fly's preferred temperature of 25 °C (77 °F), which the scientists note is also the favorite temperature of many humans, and causes the prolonged activation of cells that promote midday sleep.
“People may choose to take an afternoon nap on a hot day, and in some parts of the world this is a cultural norm, but what do you choose and what is programmed into you?” Gallio asks. “Of course, it’s not culture in flies, so there actually might be a very strong underlying biological mechanism that is overlooked in humans.”
The research was published in the journal Current Biology.
Source: Northwestern University