Sleep—it’s something that we all need but can’t seem to get enough of. Not getting enough sleep can turn the sweetest, most patient person into a short-tempered and irritable crank. When you’re tanky (tired + cranky), you might say and do things that you don’t really mean, kind of like when you’re hangry (hungry + angry). While the effects of sleep deprivation on mood and mental functioning have been well documented, less is known about how sleep loss affects aggressive behaviour.

Dr. Amita Sehgal, a professor at the Perelman School of Medicine at the University of Pennsylvania, studies behaviour and its relationship with the body’s internal clock. For her research, she uses the common fruit fly, Drosophila melanogaster, as a model to study the interactions between sleep and behaviour. In a paper published last month in eLife, Sehgal and colleagues showed that sleep deprivation in fruit flies leads to less aggression.

How do you keep fruit flies from falling asleep? By shaking them every twenty seconds for 12 hours at night. After a sleepless night, the fruit flies were pitted against either another sleep-deprived fruit fly or a well-rested fruit fly in a head-to-head battle in what the researchers called an “aggression arena”. The fights were recorded and the researchers carefully watched each video to document how frequently each fly lunged at the other—the more lunges a fly made, the more aggressive their behaviour. [This paper has some great videos of fruit flies lunging at each other in slow motion.]

In fights between a sleep-deprived fly and a well-rested fly, the well-rested fly lunged a lot more and consistently exerted dominance over its opponent. The sleep-deprived fly rarely lunged, even when it was being attacked by the other fly. In fights between two sleep-deprived flies, both flies exhibited significantly less aggressive behaviour towards each other than fights between two well-rested flies.

The researchers next asked how much sleep deprivation was required to reduce aggression. They used shaking to keep flies awake for one, three, or six hours of the night and then measured their aggression in the arena. Flies that were deprived of one or three hours of sleep behaved as aggressively as the well-rested control flies. Flies that were deprived of six hours of sleep showed suppressed aggression similar to the flies that were deprived of 12 hours of sleep.

However, the dramatic reduction in aggression following sleep deprivation was not permanent. Allowing the sleep-deprived fruit flies to recover for 24 hours or as little as six hours was enough to restore their aggression to the same levels as the non-deprived controls.

To figure out how sleep loss led to lower levels of aggression, the researchers zeroed in on octopamine, a hormone and neurotransmitter with roles in both sleep and aggression. Octopamine is the insect equivalent of norepinephrine, a neurotransmitter that, together with epinephrine, underlies the body’s “fight or flight” response and affects heart rate, sleep-wake patterns, aggression and other biological phenomena in humans. Previous work from Sehgal’s lab showed that octopamine also functions to promote wakefulness in fruit flies while another study found that male fruit flies that lack octopamine do not display any aggressive lunging behaviour.

When sleep-deprived flies were fed chlordimeform (CDM), a chemical compound that mimics octopamine, their aggression levels increased to that of the well-rested control flies. It seems that the reduced aggression in sleep-deprived flies is due in part to a disruption in octopamine signalling that occurred as a result of sleep loss.

While aggression is often described as a negative and undesirable trait in humans, it serves an important purpose in other animals, particularly with respect to food acquisition, predator defense and reproduction. The researchers tested whether the lower aggression of sleep-deprived male fruit flies affected their ability to successfully court a female fly. In these courtship tests, two male flies, both sleep-deprived, well-rested, or one of each, were put in a mating chamber with a female fly. When both males were sleep-deprived, they spent considerably less time directly competing with each other than when both males were well-rested. When a sleep-deprived and a well-rested male were paired against each other, the well-rested male was almost twice as successful in mating with the female than its sleepy rival. This is probably due to the fact that the sleep-deprived males showed less courtship aggression, such as fighting with the other male and deliberately disrupting his attempts at mating. Similar to its effect on aggression, feeding CDM to sleep-deprived males improved their mating success rates to the same levels as the well-rested males.

With our increasingly busy schedules and the invasion of cell phones, tablets and other technologies into the bedroom, sleep deficiency is now recognized as a public health concern. In a 2011 report, the US Centers for Disease Control and Prevention found that roughly one-third of adults surveyed slept less than seven hours each night.

This study adds to the large but confusing body of work on the relationship between sleep loss and changes in aggressive behaviour. Studies in humans have found that sleep deprivation does not alter or reduces aggression whereas rodent studies have found that sleep loss leads to increased aggression. One of the biggest challenges in the field is teasing apart the direct effects of sleep loss on behaviour from the indirect stress effects caused by the sleep loss. Given the growing prevalence of sleep deficiency, a more thorough understanding of the relationship between sleep loss and aggression will be important from both a scientific and public health perspective.

Until then, let’s all grab a few zzz’s and sleep on it.

Reference: Kayser MS, Mainwaring B, Yue Z, & Sehgal A (2015). Sleep deprivation suppresses aggression in Drosophila. eLife, 4 PMID: 26216041

Here’s a video of Dr. Sehgal explaining her research on PBS NOVA with Neil deGrasse Tyson: