He decided to abandon his watch and “live like an animal”. In 1961, French geologist Michel Siffre led a two-week expedition to study an underground glacier beneath the French Alps and ended up staying two months, fascinated by how the darkness affected human biology. One of the strangest effects is the ‘time-shifting’ reported by those who have spent long periods living underground without daylight. For starters, isolation messes with our sense of time. Yet some of the most profound effects of loneliness are on the mind. This may have been appropriate in our early ancestors, when being isolated from the group carried big physical risks, but for us the outcome is mostly harmful. The mechanisms behind these effects are still unclear, though what is known is that social isolation unleashes an extreme immune response – a cascade of stress hormones and inflammation. Loneliness also interferes with a whole range of everyday functioning, such as sleep patterns, attention and logical and verbal reasoning. Chronically lonely people have higher blood pressure, are more vulnerable to infection, and are also more likely to develop Alzheimer’s disease and dementia. We’ve known for a while that isolation is physically bad for us. Why does the mind unravel so spectacularly when we’re truly on our own, and is there any way to stop it? We know this not only from reports by people like Shourd who have experienced it first-hand, but also from psychological experiments on the effects of isolation and sensory deprivation, some of which had to be called off due to the extreme and bizarre reactions of those involved. For most people, prolonged social isolation is all bad, particularly mentally. We all want to be alone from time to time, to escape the demands of our colleagues or the hassle of crowds. “At one point, I heard someone screaming, and it wasn’t until I felt the hands of one of the friendlier guards on my face, trying to revive me, that I realised the screams were my own.” “In the periphery of my vision, I began to see flashing lights, only to jerk my head around to find that nothing was there,” she wrote in the New York Times in 2011. One of the most disturbing effects was the hallucinations. She endured almost 10,000 hours with little human contact before she was freed.

Accused of spying, they were kept in solitary confinement in Evin prison in Tehran, each in their own tiny cell. That summer, the 32-year-old had been hiking with two friends in the mountains of Iraqi Kurdistan when they were arrested by Iranian troops after straying onto the border with Iran. She heard phantom footsteps and flashing lights, and spent most of her day crouched on all fours, listening through a gap in the door. Sarah Shourd’s mind began to slip after about two months into her incarceration.

The long dark emergency stim