Harvard researchers help unravel the mystery of sleep

New sleep research has revealed surprising connections between the brain and the gut.

Sleep is of paramount importance among human activities – even a single night’s lack of it can impede our cognitive function, responsiveness, and overall daily performance. Despite its critical role in health and survival, the scientific understanding of sleep is still incomplete.

Enter Dragana Rogulja, a neuroscientist on his quest to unravel the basic biology of sleep.

As a self-described latecomer to science, Rogolja finds herself drawn to questions she considers “broadly interesting and easy to understand on a basic human level.”

One of these questions… What happens when we sleep?

For Rogolga, an associate professor of neurobiology at the Blavatnik Institute at Harvard Medical School, one interesting aspect of sleep is the loss of consciousness and the awareness it brings, as the outside world disappears and the inner world takes over.

In a conversation with Harvard Medicine News, Rogolja went into detail about her sleep research, which uses fruit flies and mice to explore why we need sleep and how we disconnect from the world while we sleep.

Harvard Medicine News: What do you study in the context of sleep?

Rogolja: There are two main questions my lab has been pursuing over the past several years. The first is why sleep is so important to survival. Why if you don’t sleep, you will literally die not too long after? The other question is how your brain disconnects from the environment when you fall asleep.

How are stimuli prevented from reaching your brain while you sleep? Raising the threshold for sensory arousal is essential for sleep, and we want to understand how this barrier is built around the brain. Sleep is one unified state, but it appears to have multiple components that are regulated through separate mechanisms. We want to understand those mechanisms.

HMNews: How has your research changed the way you think about sleep?

Rogolga: For a long time, scientists have been guided by the principle that sleep is brain, brain, and brain. As a result, research has largely focused on the brain in terms of looking for the reasons why sleep is necessary for survival. However, we now realize that while sleep may be for the brain, it is not just for the brain. Sleeping is a very ancient behavior that we think originated in the first animals. These animals did not have a brain. They just had a very simple nervous system.

Then, as animals became more sophisticated, so did the brain’s associated sleep objects. However, researchers have looked into the brains of sleep-deprived animals to try to find the cause of their death, and they haven’t found anything. On the other hand, clinical data shows that sleep deprivation in humans leads to all kinds of diseases in the body. To us, this really suggests that sleep is about more than just the brain.

Our research tells us that we need to stop thinking of the brain separately from the body when it comes to sleep. I am still struck by the degree to which neuroscientists tend to think the brain is superior to the body and at the top of the hierarchy. To solve neuroscience’s biggest mysteries, we need to take a more integrated approach, which is what my lab is trying to do for sleep.

We’ve found that we really need to think of the whole body to understand sleep. And that makes sense. When you go to sleep, your muscles relax and your blood circulation changes. Of course, it concerns the whole body.

HMNews: What tools do you use to study sleep?

Rogulja: Historically, a lot of sleep research has been done in humans, but these experiments tend to be limited and descriptive, because you can’t really do human experiments. However, over the past two and a half decades, scientists have realized that fruit flies sleep. And recently, we discovered that the genes that regulate sleep in flies are conserved in mice.

When I started my lab, we were only using fruit flies as a model system to study sleep, but we’ve since been able to create a mouse model as well. Fruit flies allow us to test a lot of hypotheses quickly and do large, unbiased genetic assays, and then we can test what we’ve discovered in mice flies, which are very similar to humans.

HMNews: On your 2020 cell paper, you have addressed the question of why sleep is so important to survival. what is the answer?

We found that the fruit flies that slept less had a shorter lifespan: we saw a correlation where the more sleep the flies lost, the faster they died. Interestingly, the pattern of sleep deprivation was not significant. What matters is the amount of sleep lost. There seems to be an inflection point where lack of sleep has been linked to death, telling us there may be something specific going on in the body as opposed to general wear and tear.

To further investigate this, we stained different organs in sleep-deprived flies for markers of cell damage. We found that in the gut, there was an increase in redox molecules, and the redox peak correlates with the inflection point where the flies began to die. We confirmed this finding in sleep-deprived mice. But when we gave sleep-deprived flies antioxidants or turned on antioxidant-producing genes in the gut, we found that the flies could survive with little or no sleep, which indicates that the gut is a really important target for sleep.

HMNews: Are there any potential applications for humans?

Our findings suggest that if we can prevent oxidative stress in the gut, we may be able to counteract the impact of sleep loss. This is important because so many diseases are associated with bowel dysfunction, and many of the diseases that arise when you don’t get enough sleep may actually be the result of bowel damage. We are now beginning to consider how we diagnose gut oxidative stress due to lack of sleep in humans. We want to design ‘swallowable tablets’ – tablets or tablets that you can swallow and report on the redox state in your gut, for example, by changing the color of your stool.

We’re also looking for biomarkers: molecules already circulating in the body that indicate poor sleep and gut oxidation. I have clinicians in my lab characterizing sleep-deprived mice to look for such biomarkers. We already have some molecules that are promising markers of antioxidants and they seem to decrease with antioxidant therapies. Ultimately, it may be possible to design oral nutritional supplements to reverse gut oxidation due to lack of sleep.

HMNews: You just published a new research paper in cell Explores how the brain disconnects from the environment during sleep. Tell us more.

Until now, we knew almost nothing about this. It wasn’t clear if there was one place in the brain where all sensory information is attenuated during sleep, or if there were many such places. For example, are touch and temperature processed the same way during sleep? Iris Titos, a postdoctoral researcher in my lab, has built a system that can deliver light, medium, or high levels of vibration to fruit flies.

Normally, when you use low-intensity vibrations, very few flies wake up, and when you use high-intensity vibrations, almost all the flies react. Next, we did a large-scale screen to identify the genes that control how easily flies are woken up – genes that make flies easier to wake up, and genes that basically allow flies to sleep during an earthquake.

HMNews: What did the genetic screen show?

The screen results were very interesting. We identified a gene that codes for a molecule called CCHa1. When we depleted CCHa1 in the flies, they woke up very easily — so instead of 20 percent waking up at a certain level of vibration, 90 percent woke up.

However, while CCHa1 is present in both the nervous system and the gut, it was only when we depleted it in the gut that the flies were more easily awakened. The cells in the gut that produce CCHa1 are called enteroendocrine cells, and in fact they share many properties with neurons and can even connect and communicate with neurons. These cells face the inside of the gut, and they sort of “taste” the contents of the gut.

We found that the higher the protein concentration in the diet, the more CCHa1 intestinal cells produced. This molecule then travels from the gut to the brain, where it sends signals to a small group of dopaminergic neurons that also receive information about the vibrations.

These neurons produce dopamine, which normally enhances arousal, but in this case, suppresses arousal. The vibrations dampen the activity of dopaminergic neurons, causing the flies to wake up more easily. CCHa1 produced by the gut essentially protects dopaminergic neurons against vibrations, allowing flies to be more ignorant of the environment and sleep more deeply.

We also found that the CCHa1 pathway, while important in transmitting mechanosensory information, has no effect on how easily flies wake when exposed to heat, suggesting that different sensory modalities such as vibration and temperature can be discrete. Finally, we showed that a high-protein diet also improved sleep quality in mice, making them more resistant to mechanical perturbations. We are now testing whether a similar signaling pathway is involved in mice.

HMNews: What do these results tell you?

Well, we know from other research that when animals are starving, they suppress sleep in order to forage. By contrast, when they are satiated, and especially when they are satiated with protein, they tend to sleep more. Now, we’ve shown that when there is more protein in the diet, the animals also sleep more deeply and become less responsive. This suggests that if the animals do not need to forage, they can separate from the environment and hide somewhere to sleep, which may be safer. More broadly, our study suggests that dietary choices influence sleep quality. We can now explore this link in humans to understand how diet can be manipulated to improve sleep.

HMNews: Is there anything about sleep that you think people often misunderstand?

Rogolja: One thing that I think people have to realize is that what we feel and what happens in our bodies doesn’t have to be the same. In our research, we’ve found that it’s possible to separate the feeling of sleepiness from the need for sleep — some sleep-deprived animals don’t necessarily feel sleepy, which we can tell they are because they didn’t sleep more in order to catch up on sleep afterwards. The deprivation stopped, but these animals died from lack of sleep.

This means that even if we can trick ourselves into not feeling sleepy, lack of sleep can still have negative effects on our bodies – for example, if you ingest a substance that makes you feel awake, the same amount of oxidation will occur in your gut.

People may say they’re okay with only a few hours of sleep a night, but they only mean they can get through the day. Their bodies still register a lack of sleep. We can’t really know what’s going on in our bodies as a result of sleep deprivation, and we probably need more sleep than we think.

References: “Peptide Secreted in the Gut Prevents Excitation from Sleep” By Iris Titus, Aline Yoginovich, Alexandra Vaccaro, Keshi Nambara, Pavel Gorelik, Ofer Mazur, and Dragana Rogolja, Mar. 22, 2023, Available Here. cell.
DOI: 10.1016/j.cell.2023.02.022

Reference: “Lack of Sleep Can Cause Death Through Accumulation of Reactive Oxygen Species in the Gut” by Alexandra Vaccaro, Yusuf Kaplan-Durr, Keshi Nambara, and Elizabeth A. Paulina, Cindy Lane, and Michael E. cell.
DOI: 10.1016/j.cell.2020.04.049

Additional authors on the 2023 Cell paper include Alen Juginović, Alexandra Vaccaro, Keishi Nambara, Pavel Gorelik, and Ofer Mazor of HMS.

The research was supported by the New York Stem Cell Foundation National Institutes of HealthPew Scholars Program in Biomedical Sciences.