Why it's so hard to get out of bed in winter

If you're finding it increasingly difficult to get up as the cold weather kicks in, now you have something to blame.

Scientists have found fruit flies' brains have a thermostat that keeps them asleep when it's nippy, and they think the same goes for humans.

"Temperature sensing is one of the most fundamental sensory modalities," said Marco Gallio, neurobiology expert at Northwestern University in Illinois. 

The insects have "absolute cold" receptors in their antennae that are activated when the temperature drops below 25C - cold by their standards. They send signals to a group of neurons in the brain connected to the fly's circadian network, which governs when it wakes and sleeps.

When it's cold, the neurons - normally activated by morning light - stay quiet. 

"This helps explain why - for both flies and humans - it is so hard to wake up in the morning in winter," said Dr Gallio.

It might also explain why we find it so hard to go to sleep when it's hot. 

Dr Gallio said while so far they've only found the link in fruit flies, it makes sense something similar would work in humans too - after all, we are both "creatures of comfort and are continually seeking ideal temperatures".

"The principles we are finding in the fly brain - the logic and organisation - 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."

Despite the fact we spend between a quarter and a third of our lives asleep, science knows remarkably little about it. For example, it was only in recent years that researchers discovered how blue light affects our circadian rhythms and sleep.

"The ramifications of impaired sleep are numerous - fatigue, reduced concentration, poor learning and alteration of a myriad of health parameters - yet we still do not fully understand how sleep is produced and regulated within the brain and how changes in external conditions may impact sleep drive and quality," said Michael Alpert, a postdoctoral fellow in Dr Gallio's lab.

The research was published in journal Current Biology.