Being the social animals that we are, humans need company almost as much as they need water, air, and nourishment. Past studies have shown enduring isolation puts us at risk of physical harm in more ways than one.

A new study by researchers from the University of Vienna in Austria and the University of Cambridge in the UK has now found eight hours of loneliness can sap energy and increase fatigue as much as going eight hours without food in certain people.

The team's lab test and field experiment showed people who live alone or who particularly enjoy social interactions are the most likely to be affected by a lack of company. What's more, it seems as though the reduction in energy is the result of changes in the body's homeostatic response: a sort of balancing act, where the lack of social connection triggers a biological reaction.

"In the lab study, we found striking similarities between social isolation and food deprivation," say the first authors of the study, psychologists Ana Stijovic and Paul Forbes from the University of Vienna in Austria.

"Both states induced lowered energy and heightened fatigue, which is surprising given that food deprivation literally makes us lose energy, while social isolation would not."

For the lab study, 30 female volunteers were examined on three separate days of eight hours each: one day without social contact, one day without food, and one day without either social contact or food. The participants gave feedback on their stress, mood, and fatigue, while heart rate and salivary cortisol levels (standard stress indicators) were also measured.

The field experiment involved 87 participants living in Austria, Italy, or Germany, and covered periods of COVID-19 lockdown measures between April and May 2020. Those involved had spent at least eight hours in isolation, and were asked to answer questions via a smartphone app similar to those asked in the lab test: on stress, on mood, and on fatigue.

While the field experiment didn't involve food, its results – lower levels of energy after isolation – match up with the lab work, suggesting that the comparison between going without social interaction and going without sustenance is a valid one. The real-world test was also where those living alone and the more sociable were shown to be most affected. Their reported energy levels dropped on days where they interacted with no-one compared to days with some brief social interactions – an effect not seen in less sociable participants.

"The fact that we see this effect even after a short period of social isolation suggests that low energy could be a 'social homeostatic' adaptive response, which on the long run can become maladaptive," says psychologist Giorgia Silani, from the University of Vienna.

So as the time in isolation extends, the damage is likely to get worse: previous studies have compared loneliness to public health problems such as obesity, suggesting that there's a significant risk of premature death due to being isolated socially.

Earlier research has also shown evidence of a feedback loop, where a lack of social engagement makes us less likely to want to get out into the world and make connections – a sort of loneliness spiral that's increasingly hard to get out of.

We also know that spending time alone can be beneficial for certain people in terms of their well-being. Future research across larger and more diverse groups of participants will be able to examine these associations further.

"It is well-known that long-term loneliness and fatigue are related, but we know little about the immediate mechanisms that underlie this link," says Silani.

The research has been published in Psychological Science.