In the middle of the night, the world can sometimes feel like a dark place. Under the cover of darkness, negative thoughts have a way of drifting through your mind, and as you lie awake, staring at the ceiling, you might start craving guilty pleasures, like a cigarette or a carb-heavy meal.
Plenty of evidence suggests the human mind functions differently if it is awake at nighttime. Past midnight, negative emotions tend to draw our attention more than positive ones, dangerous ideas grow in appeal and inhibitions fall away.
Some researchers think the human circadian rhythm is heavily involved in these critical changes in function, as they outline in a new paper summarizing the evidence of how brain systems function differently after dark.
Their hypothesis, called 'Mind After Midnight', suggests the human body and the human mind follow a natural 24-hour cycle of activity that influences our emotions and behavior.
In short, at certain hours, our species is inclined to feel and act in certain ways. In the daytime, for instance, molecular levels and brain activity are tuned to wakefulness. But at night, our usual behavior is to sleep.
From an evolutionary standpoint this, of course, makes sense. Humans are much more effective at hunting and gathering in the daylight, and while nighttime is great for rest, humans were once at greater risk of becoming the hunted.
According to the researchers, to cope with this increased risk our attention to negative stimuli is unusually heightened at night. Where it might once have helped us jump at invisible threats, this hyper-focus on the negative can then feed into an altered reward/motivation system, making a person particularly prone to risky behaviors.
Add sleep loss to the equation, and this state of consciousness only becomes more problematic.
"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," says neurologist Elizabeth Klerman from Harvard University.
"My plea is for more research to look at that, because their health and safety, as well as that of others, is affected."
The authors of the new hypothesis use two examples to illustrate their point. The first example is of a heroin user who successfully manages their cravings in the day but succumbs to their desires at night.
The second is of a college student struggling with insomnia, who begins to feel a sense of hopelessness, loneliness and despair as the sleepless nights stack up.
Both scenarios can ultimately prove fatal. Suicide and self-harm are very common at nighttime. In fact, some research reports a three-fold higher risk of suicide between midnight and 6:00 am compared to any other time of day.
A study in 2020 concluded that nocturnal wakefulness is a suicide risk factor, "possibly through misalignment of circadian rhythms."
"Suicide, previously inconceivable, emerges as an escape from loneliness and pain, and before the costs of suicide are considered the student has acquired the means and is prepared to act at a time when no one is awake to stop them," the authors of the 'Mind After Midnight' hypothesis explain.
Illicit or dangerous substances are also taken more by people at night. In 2020, research at a supervised drug consumption center in Brazil revealed a 4.7-fold greater risk of opioid overdose at night.
Some of these behaviors could be explained by sleep debt or the cover that darkness offers, but there are probably nighttime neurological changes at play, too.
Researchers like Klerman and her colleagues think we need to investigate these factors further to make sure we are protecting those most at risk from nighttime wakefulness.
To date, the authors say no studies have examined how sleep deprivation and circadian timing impact a person's reward processing.
As such, we don't really know how shift workers, such as pilots or doctors, are coping with their unusual sleep routine.
For six hours or so a day, we know surprisingly little about how the human brain works. Whether asleep or awake, the mind after midnight is a mystery.
The study was published in Frontiers in Network Psychology.