Scientists continue to discover ways in which a lack of sleep affects our mental and physical health – now a new study reveals that a serious lack of shut-eye can even influence the way we see other people.

When we've gone without sleep, we spend less time fixing our gaze on other people's faces, the study shows. As that's a crucial part of reading social cues from those around us, our relationships could potentially suffer.

What's more, after sleep loss, angry faces appear to us to be less trustworthy and less healthy, while neutral or fearful faces come across as less attractive, compared to when we've had a full night's sleep.

"Since facial expressions are crucial to understanding the emotional state of others, spending less time fixating on faces after acute sleep loss may increase the risk that you interpret the emotional state of others inaccurately or too late," says sleep researcher Lieve van Egmond from Uppsala University in Sweden.

The study authors recruited 45 participants who went through a night without sleep, and another with 8 hours of slumber, separated by at least a week. In each case, eye-tracking sensors were used the morning after to monitor the gaze of the subjects as they looked at images of faces.

A mix of expressions were shown on the faces: happy, angry, fearful, and neutral. Participants were also asked to rate the attractiveness, trustworthiness and healthiness of the faces they saw.

When it came to face fixation, there was a drop in duration between 6.3-10.6 percent after sleep loss, and this drop happened irrespective of the emotion being shown. Overall, faces were rated as less trustworthy and less attractive after a night without sleep.

"The finding that sleep-deprived subjects in our experiment rated angry faces as less trustworthy and healthy-looking and neutral and fearful faces as less attractive indicates that sleep loss is associated with more negative social impressions of others," says neuroscientist Christian Benedict from Uppsala University.

"This could result in less motivation to interact socially."

It's perhaps no surprise that a lack of sleep makes us less likely to want to engage with others, but the study adds some interesting data to the mix. Negative social impressions of people after sleep loss may lead to social withdrawal by those with sleep issues, suggest the researchers.

The team behind the study also hypothesizes that going without sleep and, as a result, fixating less on faces could cause problems when it comes to judging other people's emotional states – which is a key part of keeping social connections going.

We already know that poor sleep means we're less able to pay attention and slower to interpret emotions, which may be contributing factors here. However, studies with larger sample sizes over a longer time will be needed to really dig into what's happening.

"Our participants were young adults," says van Egmond. "We do not know whether our results are generalizable to other age groups. Moreover, we do not know if similar results would be seen among those suffering from chronic sleep loss."

The research has been published in Nature and Science of Sleep.