When Oliver Sacks began seeing Bill Hayes in 2009, he had never been in a relationship. He wasn’t out as a gay man and hadn’t had sex for thirty-five years. Sacks, the celebrated author and neurologist, was almost thirty years older than Hayes, who had moved to New York from San Francisco after the sudden death of his partner. The two visited the Museum of Natural History and went for walks in the Bronx botanical garden, where Sacks could expatiate on every species of fern. When Hayes gave Sacks a long, exploratory kiss on his seventy-sixth birthday, the older man looked utterly surprised. ‘Is that what kissing is?’ he asked. ‘Or is that something you’ve invented?’

Hayes’s luminous memoir, Insomniac City: New York, Oliver, and me, is full of such startling questions. For Sacks’s mind – erudite, deeply scientific, yet with a childlike sense of wonder – must now process the mysteries of love. He caresses his lover’s biceps: ‘they’re like ... beautiful tumours’. As he watches Hayes do his daily push-ups, he counts them by naming the elements: titanium, vanadium, chromium. ‘Wouldn’t it be nice if we could dream together?’ he asks Hayes one night in bed.