Scientists say that SLEEPY people tend to be selfish and tight-fisted.
Lack of kip reduces brain activity which is vital for feeling generous, sociable.
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Researchers at the University of California Berkeley discovered that people are less likely to give money or help strangers when they’re tired.
Even just one hour of shut-eye can result in a drop of donations to worthy causes.
The NHS says adults should get seven to nine hours in bed per night – but three quarters of Brits sleep less than they should.
Study author Professor Matthew Walker said: “Helping is a core feature of humankind so this demonstrates that a lack of sleep degrades the fabric of human society itself.
“How we operate as a social species seems profoundly dependent on how much sleep we are getting.”
Co-author Dr Eti Ben Simon added: “If you’re not getting enough sleep, it doesn’t just hurt your own wellbeing – it hurts your entire social circle, including strangers.”
The study, in the journal PLOS Biology, quizzed people after one night’s bad sleep and over a longer period of time – and counted charity donations after the clocks went back.
When they are tired, people are 7.5% less likely to help strangers or friends than when they are well-rested.
Three out of four people – 78 per cent – experienced the drop in helpfulness.
An analysis of millions of US charity donations revealed a 11% drop in their value after the clocks were reset for daylight savings. This is in comparison to the previous month.
Losing just a single hour’s sleep when the clocks changed was enough to make a whole society less generous with their cash, the scientists said.
MRI scans revealed less activity in parts of the brain called the social cognition network when someone was drained – and this made them meaner.
Dr Liz Halstead, from the Psychology and Human Development department at University College London, was not involved in the study but said: “This study really shows how important sleep is in human social function.
“We know insufficient sleep affects mood, memory and decision-making abilities – so it can have a big impact on people.
“Research shows that over 50 per cent of people do not get the right amount of sleep for them and that will have big implications for their interactions with other people.”