It’s not just the outside world that is shrouded in darkness at night. Scientists are making the observation that our minds are more susceptible to negative thinking during the night than in the daytime, and this could have significant consequences for our mental health. In a new study, researchers have presented this effect under the ominous name of ‘Mind After Midnight’ to raise awareness and call for more research into the physiological and psychological processes that start to take over our brains deep into the night.

The human brain thrives on light

Unlike rats and owls, humans are not nocturnal creatures. We evolved to be diurnal, or active during the day, and this is easy to prove by studying the circadian rhythm — the 24-hour cycle that determines wakefulness and sleep — which, in humans, is obviously geared toward sleeping in the dark. The brain can tell when it’s nighttime based on the amount of light over time it detects via the eyes.

When it’s dark, the brain floods the body with hormones that lower blood pressure, stress levels, body temperature, and other things that generally make us sleepy and prime us for slumber. On the flip side, the morning sunshine flips chemical switches that make us more alert and wakeful.

When this natural rhythm is disrupted, such as by staying up late at night, a host of deleterious consequences can occur, including sleep disorders. Over time, it can make it hard to fall asleep and leave you constantly fatigued throughout the day, as well as affecting memory, mood, physical health, and overall function.

But while most research has focused on examining what poor nightly sleep does to us the next day, not much attention has been given to what actually happens in those instances when we’re wide awake in the middle of the night.

“The basic idea is that from a high level, global, evolutionary standpoint, your internal biological circadian clock is tuned towards processes that promote sleep, not wakefulness, after midnight,” says Elizabeth Klerman, MD, PhD, an investigator in the Department of Neurology at Massachusetts General Hospital, Professor of Neurology at Harvard Medical School and the senior author of the paper.

“There are millions of people who are awake in the middle of the night, and there’s fairly good evidence that their brain is not functioning as well as it does during the day,” she added. “My plea is for more research to look at that, because their health and safety, as well as that of others, is affected.”

Klerman and colleagues reviewed a number of studies and publicly available statistics showing how staying active after dark can affect our brain systems and, in turn, our behavior. The evidence they’ve gathered thus far suggests that staying awake late at night makes us more biased towards negative emotions and more prone to taking risks that may endanger our physical integrity.

For instance, suicides are much more likely to occur during nighttime hours than during the day. Homicides and other violent crimes are most common at night, as is the use of illicit drugs, as well as unhealthy eating habits like snacking on carb-rich foods in the middle of the night.

It seems like a lot of unhealthy choices come out at night to haunt us. This observation has prompted Klerman and colleagues to propose a new hypothesis called the ‘Mind After Midnight’, which argues there may be a biological basis for all of these reported nighttime negative effects.

The idea is that things like attentional biases, negative affect, altered reward processing, and prefrontal disinhibition interact to promote behavioral dysregulation and even psychiatric disorders. The researchers cite studies that show how the circadian rhythm influences neural activity over the course of 24 hours, thereby affecting our moods and the way we interact with the world. For instance, research shows that positive affect, that is the tendency to view information in a positive light, is at its highest during the morning, whereas negative affect is highest at night.

Research also shows that the human brain produces more dopamine at night, an important neurotransmitter that plays a role in many important body functions, including movement, memory, and pleasurable reward and motivation. This inflow of dopamine can hijack the reward and motivation system in the brain, making us more prone to risky and impulsive behavior, whether it’s snacking on a huge bucket of ice cream at 12:00 AM or shooting heroin at night after resisting the cravings during the day.

Almost everyone has probably had to face the nighttime blues at least at some point in their lives, a weird dark hour when your worldview becomes narrower and more negative. The world is suddenly much smaller than it actually is and it just sucks. Klerman herself is no exception.

“While part of my brain knew that eventually I would fall asleep, while I was lying there and watching the clock go tick tick tick—I was beside myself,” she recalls.

“Then I thought, ‘What if I was a drug addict? I would be out trying to get drugs right now.’ Later I realized that this may be relevant also if it’s suicide tendencies, or substance abuse or other impulse disorders, gambling, other addictive behaviors. How can I prove that?”

For now, the Mind After Midnight is just an unvalidated hypothesis, but a concerning one that deserves further attention. Ironically, though, in order to investigate it, there would have to be some researchers who would need to be working after midnight to supervise test subjects. This may include, for instance, taking fMRI images of the brains of volunteers’ with disrupted sleep cycles.

“Most researchers don’t want to be paged in the middle of the night. Most research assistants and technicians don’t want to be awake in the middle of the night,” Klerman concedes.

“But we have millions of people who have to be awake at night or are awake at night involuntarily. Some of us will have to be inconvenienced so we can better prepare them, treat them, or do whatever we can to help.”