A study conducted by neurobiologists from Northwestern University made it possible to elucidate the mechanisms underlying this
 phenomenon.
Their study on fruit flies, has found a thermometer circuit can transfer information about cold temps from their antennas right to the
 brain.
They show how, through this circuit, seasonally cold and dark conditions can inhibit neurons within the fly brain that promote 
activity and wakefulness, particularly in the morning.
“This helps explains why -- for both flies and humans -- it is so hard to wake up in the morning in winter,” said Marco Gallio, 
associate professor of neurobiology in the Weinberg College of Arts and Sciences. “By studying behaviors in a fruit fly, we can 
better understand how and why temperature is so critical to regulating sleep.”
The study, led by Gallio and conducted in Drosophila melanogaster, was published in the journal Current Biology.
The paper describes for the first time “absolute cold” receptors residing in the fly antenna, which respond to temperature only 
below the fly’s “comfort zone” of approximately 77 degrees Fahrenheit.
Having identified those neurons, the researchers followed them all the way to their targets within the brain. They found the main
 recipients of this information are a small group of brain neurons that are part of a larger network that controls rhythms of activity 
and sleep. When the cold circuit they discovered is active, the target cells, which normally are activated by morning light, are shut
 down.
“Temperature sensing is one of the most fundamental sensory modalities,” said Gallio, whose group is one of only a few in the 
world that is systematically studying temperature sensing in fruit flies. “The principles we are finding in the fly brain -- the logic 
and organization -- may be the same all the way to humans. Whether fly or human, the sensory systems have to solve the same 
problems, so they often do it in the same ways.”

来源：Study Finds，news.northwestern.edu