The longer someone stays awake, the more likely they'll start getting tired as their brain needs sleep. But how the brain senses that need for sleep hasn't always been clear. Now, Johns Hopkins Medicine researchers have shown in fruit flies that certain groups of brain cells called astrocytes sense electrical activity in different regions of the brain and use these signals to facilitate the process of falling asleep. The more activity that they detect, the stronger the need-for-sleep signals become, until they trigger a release mechanism that pushes sleep.