Why humans and other animals sleep when we do has long been a mystery—an enigma that’s hampered finding ways to treat sleep disorders. To help solve this puzzle, Johns Hopkins neurologist Mark Wu, who treats patients with sleep disorders in the clinic and studies these problems in the lab, turned to the quintessential biological model: the fruit fly.

Searching for sleep-regulating cells in the insects’ brains, Wu’s team used genetic engineering to turn on small numbers of neurons in more than 500 Drosophila strains. They then measured how these flies slept when these neurons fired. Several strains continued to sleep for hours even after they turned off the neurons, suggesting that the researchers triggered sleep drive in these flies, which led to the persistent sleepiness.

Using fluorescent microscopy, the scientists then examined the fly brains to specifically pinpoint the identity and location of the sleep drive-inducing cells, which were genetically engineered to glow green. They were found in a structure called the ellipsoid body. 

To further confirm that they’d found the right cells, the researchers blocked the neurons from firing by genetically engineering them to make tetanus toxin, which silenced the cells. The flies with the silenced neurons slept on their normal schedule, but when they were deprived of sleep during the night by mechanically shaking their vial houses, they got about 66 percent less “rebound sleep” compared to control flies, suggesting that they felt less sleepy after sleep deprivation.

Next, the researchers tested how these special neurons behaved on their own in awake, sleeping or sleep-deprived fruit flies. They used tiny electrodes to measure the firing of these cells in well-rested, awake fruit flies, in fruit flies that were an hour into their sleep cycle, and in fruit flies after 12 hours of sleep deprivation.

In the well-rested fruit flies, the neurons fired only about once per second and were the least active. In the sleeping fruit flies, the neurons fired almost four times a second. In the sleep-deprived fruit flies, the neurons were the most active, firing at about seven times per second.

“These neurons have higher firing rates the more sleep-deprived the fruit flies were and firing of these neurons puts flies to sleep, suggesting that we’ve identified the key cells responsible for sleep drive,” says Wu.

Further investigation by Wu’s team suggests that sites on the neurons’ surfaces that release sleep-promoting neurotransmitters increase in size and number when the flies are sleep-deprived, allowing a flexible system for triggering sleep when the insects most need rest.

“Figuring out how sleep drive works should help us one day figure out how to treat people who have an overactive sleep drive that causes them to be sleepy all the time and is resistant to current therapies,” Wu says.