The longer someone stays awake, the more likely they’ll start to nod off as their brain needs sleep. But how the brain senses that need for sleep hasn’t always been clear. Now, neurologists like Mark Wu, are investigating how this brain function works. He and his research team found that in fruit flies, certain groups of brain cells sense electrical activity in different regions of the brain and use these signals to start falling asleep. The cells, pictured here are called astrocytes. The more activity that they detect, the stronger the need-for-sleep becomes.

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