Sleep is widely recognized as one of life's essential processes, providing powerful benefits in physical health, mental health and even mortality.
But did you know that sleepless nights can also lead to selfish behavior?
Insufficient sleep affects how likely a person is to help someone, according to new research published in the journal PLOS Biology on Tuesday.
Researchers from the University of California, Berkeley, conducted three studies in the United States looking at this "selfish" effect, analyzing changes in neural activity and behavior benefiting others, and found it was prevalent even after a small loss of sleep.
Research scientist Eti Ben Simon and Matthew Walker, a professor of neuroscience and psychology at UC Berkeley and director of the university's Center for Human Sleep Science, were the study leads. They told CNN that this finding was most surprising.
"Even just an hour of sleep loss was more than enough to influence the choice to help another," said Ben Simon, a postdoctoral fellow of psychology at the Center for Human Sleep Science. "When people lose one hour of sleep, there's a clear hit on our innate human kindness and our motivation to help other people in need."
By looking at a database of 3 million charitable donations between 2001 and 2016, Ben Simon, Walker and their colleagues saw a 10% drop in donations following Daylight Saving Time. This drop was not seen in states that don't follow the one-hour transition forward.
In the second study, the researchers used functional magnetic resonance imaging to look at the brain activity of 24 people after eight hours of sleep and after a night of no sleep. The prosocial neural network -- the areas of the brain associated with theory of mind -- was less active after sleep deprivation, this study found.
Theory of mind is the ability to consider other people's needs, states and emotions, which typically develops in early childhood with socialization.
"Sleep has been consistently shown to affect our mood and our cognitive functioning, and thus, it also likely affects how we relate to others," said Dr. Ivana Rosenzweig, a sleep physician and consultant neuropsychiatrist at Guy's and St Thomas' Hospital in the UK, who was not involved in the study.
In the third study, which measured the sleep of more than 100 people across three to four nights, researchers unexpectedly found that quality of sleep was more important than the quantity of sleep when it came to measuring selfishness. The team assessed levels of selfishness based on responses to questionnaires that had been completed by study participants. Sleep quantity and quality both typically influence emotional and social behavior, so the team was expecting to find an effect from both, Ben Simon told CNN.
"These findings could suggest that once sleep duration rises above some basic nominal amount, then it appears to be the quality of that sleep that is most critical for aiding and supporting our desire to help other people," she explained.
Rosenzweig, who heads the Sleep and Brain Plasticity Centre at King's College London, told CNN that this "points to the importance of sleep of good quality and quantity to the overall balanced social and cognitive functioning, including altruism."
She added that, while the methodological limitations mean that the mechanisms -- if any -- that underpin these findings cannot be judged, the study is original and creative, "which raises some stimulating questions and asks for replication of findings in a more focused and targeted way."
More than half of all people in developed countries say they get insufficient sleep during the work week, which Walker calls a "global sleep-loss epidemic." Extensive research has already shown links to mental health disorders such as anxiety and depression, as well as physical ailments such as diabetes and obesity.
Now, as evidence increasingly becomes available on its negative impact on social behavior, it could have consequences for society today, Walker added.
Ben Simon and Walker hope their research will allow people to reclaim a full night of sleep without embarrassment or the stigma of laziness.
"(Sleep loss) radically alters how we are as social, emotional beings, which you could argue is the very essence of human interaction and what it means to live a fulfilling, meaningful human existence," Walker said.