When I was a graduate student, my colleagues and I studied how losing one night of sleep affects a person's ability to manage their emotions. Once a week, typically on a Friday evening, I would stay up all night to monitor our participants and ensure that they followed the protocol. At about noon the next day, we would all stumble out of the laboratory, exhausted and eager to get home and rest.

Two months into the experiment, I was in my car at a traffic light when a silly love song started playing on the radio. Suddenly, I was crying uncontrollably. I remember feeling surprised at my reaction. It then hit me that I was not just studying sleep deprivation—I had become part of the study. Weeks of missed sleep had taken their toll, and I was no longer in control of my emotions.

That research project, and many that have followed since, demonstrated a strong and intimate link between better sleep and emotional health. In healthy individuals, good-quality sleep is linked with a more positive mood—and it takes just one night of sleep deprivation to trigger a robust spike in anxiety and depression the following morning. Moreover, people who suffer from chronic sleep disruption tend to experience daily events as more negative, making it hard to escape a gloomy mindset. Indeed, in a national sleep survey, 85 percent of Americans reported mood disruption when they were not able to get enough sleep.

Studies from our lab and others are now beginning to illuminate just how a lack of sleep frays the inner fabric of our mind. One of its many impacts is to disrupt the brain's circuitry for regulating emotions.

For decades researchers and medical professionals considered sleep loss a by-product or symptom of another, more “primary” condition, such as depression or anxiety. In other words, first comes the anxiety, and then sleep loss follows. Today we know that this order can be reversed. In fact, sleep loss and anxiety, depression or other mental health conditions may feed into one another, creating a downward spiral that is exceedingly difficult to break.

Much evidence in this area comes from chronic sleeplessness or insomnia. People who suffer from insomnia are at least twice as likely to develop depression or anxiety later in life, compared with individuals who sleep well. For instance, a study that followed 1,500 individuals—some with insomnia and others without—found that chronic sleeplessness was associated with a three times greater increase in the onset of depression a year later and twice the increase in the onset of anxiety.

Insomnia symptoms also raise the risk of developing post-traumatic stress disorder and track closely with suicidal behavior among at-risk individuals. They often precede a mood episode in people with bipolar disorder. Even after adequate treatment for depression or anxiety, people who continue to suffer from sleep difficulties are at greater risk of relapse relative to those whose sleep improves. Understanding sleep's role in this pattern could unlock insights for helping to prevent and treat many emotional and mental disorders.

Older research already revealed that sleep loss can precede serious mental health symptoms in otherwise healthy individuals. In studies conducted mostly in the 1960s, volunteers who stayed awake for more than two nights reported difficulties forming thoughts, finding words and composing sentences. They suffered from hallucinations, such as seeing inanimate objects move or experiencing the sensation of another's touch despite being alone. After three days without sleep, some participants became delusional and paranoid. They believed they were secret agents or that aliens were contacting them. (If that sounds like a psychotic episode, that's because it is.) After five days, several participants entered a state resembling a full-blown clinical psychosis and were unable to fully comprehend their circumstances.

In one study from 1947, volunteers from the U.S. military attempted to stay awake for more than four nights. A soldier who was described by his friends as quiet and reserved became extremely aggressive after three nights without sleep. He provoked fights and insisted he was on a secret mission for the president. Eventually he was forcibly restrained and dismissed from the experiment. Six others exhibited outbursts of violence and persistent hallucinations. In all cases, after sleeping for an entire day, the soldiers behaved normally again and had no recollection of the earlier mayhem. In another study, in which participants stayed awake for four nights, researchers were unprepared for the “frequent psychotic features” they encountered, such as intense hallucinations and paranoid delusions.

Given these destructive effects, studies of prolonged sleep loss are now considered to be unethical, but they still offer a powerful reminder of just how sleep-dependent our minds and mental health truly are.

Even with these startling results, scientists have been skeptical about the consequences of restless nights, particularly given that (fortunately) few of us endure such extreme deprivation. That's where the newest wave of research comes in. In recent years a neuroscientific explanation has emerged that is beginning to illuminate what it is about sleep, or the lack of it, that seems to have a direct link to our emotions.

Whenever we face a nerve-wracking or emotionally intense challenge, a hub deep in the brain called the amygdala kicks into gear. The amygdala can trigger a comprehensive whole-body response to prepare us for the challenge or threat we face. This flight-or-fight response increases our heart rate and sends a wave of stress hormones rushing into our bloodstream. Luckily, there's one brain region standing between us and this cascade of hyperarousal: the prefrontal cortex, an area right behind the middle of our eyebrows. Studies show that activity in this region tends to dampen, or downregulate, the amygdala, thus keeping our emotional response under control.

In studies where my colleagues and I deprived healthy volunteers of one night of sleep, they discovered that the activity of the prefrontal cortex dropped drastically, as measured using functional magnetic resonance imaging (fMRI). Moreover, the neural activity linking the amygdala and the prefrontal cortex became significantly weaker. In other words, both the region and the circuit meant to keep our emotional reactions under control are essentially out of order when sleep is disrupted. Other studies have found that this profile of neural impairment can occur in people after they experience just one night of sleep deprivation, in people who are habitual short sleepers, or when participants' sleep is restricted to only four hours a night for five nights.

This impairment can be so robust that it blurs the lines around what people consider emotional. For example, when my colleagues and I exposed participants to neutral and emotional pictures (think bland images of commuters on a train versus photographs of children crying), fMRI revealed that the amygdala responded differently to these prompts when people were well rested. But after losing a night of sleep, a person's amygdala responded strongly to both kinds of images. In other words, the threshold for what the brain deems emotional became significantly lower when the amygdala could not act in concert with the prefrontal cortex. Such impaired emotional control makes us more vulnerable to anxiety and poor mood, so that even silly love songs can trigger sobbing.

The effects on the amygdala, the prefrontal cortex and the circuitry between the two may have many other consequences as well. In January we published findings that show that changes in this brain circuit, together with other regions involved in arousal, relate to increases in blood pressure after one night of sleep loss. The brain-level mechanisms my colleagues and I have observed may contribute to changes that negatively affect the entire body, increasing the risk for hypertension and cardiovascular disease.

Stepping back, it becomes clear that—like our physical well-being—mental and emotional health rely on a delicate balance. Myriad choices we make throughout the day and night maintain that balance. Even a single sleepless night can therefore do damage. We need to be mindful of this reality, for both ourselves and one another. Inevitably we all miss out on sleep from time to time. But our societies should critically examine structures—such as work norms, school cultures, and the lack of support for parents or other caregivers—that prevent people from getting enough rest. The science of sleep and mental health suggests that failing to address those problems will leave people vulnerable to serious harm.