A couple of sleepless nights can cloud a person's moral judgement, study findings suggest.
The brain needs rest to function
Army researchers found soldiers struggled to make snap decisions in emotionally charged situations after being deprived of sleep for two nights.
The authors say this could be important for other professions, including doctors, who have broken sleep and need to make quick decisions in a crisis.
The US work is published in the latest edition of the journal Sleep.
Lack of shut-eye
It is well established that poor sleep can impair many functions, including concentration and memory.
Some people say they need as little as three hours of sleep in every 24 hours to feel rested, while others need 11 hours.
Experts generally advise people get about eight hours of solid sleep per night to be on top form.
Lead researcher Dr William Killgore, of the Walter Reed Army Institute of Research, stressed that the findings did not mean that sleep deprivation leads to a decline in "morality" or in the quality of moral beliefs.
"Our results simply suggest that when sleep deprived, individuals appear to be selectively slower in their deliberations about moral personal dilemmas relative to other types of dilemmas," he said.
The dilemmas used in the study were hypothetical scenarios, rather than actual events.
The researchers asked 26 healthy soldiers to judge whether a given course of action would be "appropriate" or "inappropriate" in each of the test situations, ranging from minor inconsequential ones to serious dilemmas where the decision could theoretically harm another person.
In general, the soldiers found it harder to perform the task when they were sleep deprived and had been awake continuously for 53 hours.
Some even changed their view of what was morally acceptable after they had been awake for two days.
The US military currently is investigating how little sleep soldiers can safely get by on and whether drugs can help them to stay alert for longer without sleep.
Neil Stanley of the British Sleep Society said: "We know that being tired impairs judgement generally. This work suggests it affects moral judgement too.
"You react differently and situations can escalate. Your emotions can be out of kilter and you are either close to tears or a zombie."
He said there was a worrying trend towards a 24-hour society, with people burning the candle at both ends.
"We should not accept tiredness as a normal state," he said.
He said the longest stint ever recorded was a person who went 11.5 days without sleep.
Sleep deprivation is exceedingly common, not just among shift workers.
Allen Davey, director of the British Snoring and Sleep Apnoea Association, estimates at least a quarter of people in the UK do not get enough sleep because a family member snores.