Lack of proper sleep has been linked to all sorts of adverse health effects, from obesity, diabetes, and heart disease, to depression and poor mental health. Even more dire is the fact that sleep deprivation is lethal, although the reasons for that had been unclear— until now. 

A recent study shows that sleep-deprived fruit flies die because of the accumulation of reactive oxygen species in the small and large intestines. The findings point to clearing reactive oxygen species from the gut as a possible key function of sleep and get us closer to understanding why we need bed rest to survive.

Reactive oxygen species, or ROS for short, are volatile compounds that our cells produce normally. They have important functions in the body, but if they’re not promptly removed, they can damage and kill nearby cells. When ROS builds up, cells enter a state known as oxidative stress in which they become damaged beyond repair. 

“We think that [ROS accumulation in the gut] could explain a lot of the problems that occur in the brain during sleep deprivation, because oxidative stress in the gut is linked to neurodegenerative diseases,” says corresponding author Dr. Dragana Rogulja, assistant professor of neurobiology at Harvard Medical School, who studies fruit flies to understand why humans sleep.

Fruit flies are a great model for studying human sleep. Like humans, flies are active during the day and inactive at night, and they have an internal biological clock that tells them when to sleep and when to be awake. 

Like many other animals, fruit flies die when they are deprived of sleep. Rogulja’s lab did several experiments to figure out why.

Her team first had to come up with an efficient way to keep the flies awake for days. They engineered their fruit flies so that the brain cells that normally keep them awake during the day stayed active when the temperature was above 29°C. In this warm environment, the flies stayed awake until they died 10 days later.

“That was a real shocker to me,” says Rogulja. “And it was really cool because it happened at a predictable time.”

They examined the bodies of the flies to see if  they could find any damage that could be causing them to die. They looked at many organs and only one showed a major difference. The flies that hadn’t slept for 10 days had increased levels of ROS in their gut. “It was just like everywhere. And it was so obvious. It totally freaked us out,” says Rogulja.

The authors used a probe that fluoresces red when ROS is present . ROS is undetectable the gut after 1 day of sleep deprivation (left) but progressively accumulates and is evident at days 7 (middle) and day 10 (right).

The gut of the flies that hadn’t slept for 10 days glowed with an intense fuchsia color that indicated ROS was building up (see the picture on the right, above). The guts of the flies that were sleeping normally weren’t glowing. The result was so clear that it made Rogulja and her team a bit skeptical.

They tried keeping the flies awake in other ways, like shaking the tubes they were on every 2 seconds, or mutating the genes that control their sleep. The result was the same: the sleep-deprived flies were dead within days, and their guts, but not any other organ, were glowing fuchsia with ROS. 

They also tried similar experiments with mice, sleep-depriving them for 5 days and then looking at their organs for signs of oxidative stress. As with the fruit flies, ROS accumulated in the gut of sleep-deprived mice, with no signs of ROS in the brain or other organs. 

Next, they tried giving antioxidants to the sleep-deprived flies to see if getting rid of ROS could spare them from dying. The antioxidants eliminated ROS and the flies survived past 10 days, even though they weren’t sleeping. 

But if you are sleep-deprived, this doesn’t mean you can just reach for the antioxidant pills. “We’re not saying that you can take an antioxidant and not sleep or live longer,” says Rogulja.

Reactive oxygen species also have important functions in the body so you don’t want to get rid of them altogether. “ ROS are not just some damaging molecules,” Rogulja explains. “They regulate cell turnover in the gut and you also make ROS as defense against microbes. You cannot shut this program down without also inflicting some damage,” she says.

Sleep originates in the brain, and the consequences of not getting enough slumber, like being unable to remember or learn new things, are related to possible impairments in cognitive function. However, Rogulja and her team only found obvious signs of oxidative stress in the gut. Rogulja explained that this might be because brain cells don’t actively replace themselves like gut cells do, so brain cells need an active antioxidant mechanism that protects them. “It makes sense to have this super tight antioxidant production in the brain,” she says. “But in the gut, you cannot prevent ROS.”

Although there were no obvious signs of damage in the brain, this does not mean the brain is spared of trouble from sleep deprivation. Rather, these results seem to support the notion that sleep might not be all about the brain. For example, the levels of melatonin, the hormone humans make in preparation for sleep, is 400 times higher in the gut than in the brain. “[The gut is] a remarkable organ,” Rogulja says. I thought of it as some boring thing. And now that I learned about it, I could not have been more wrong.”