The patient was a 37-year-old man who had been physically abused as a boy by his schizophrenic mother, often while he lay in bed trying to fall asleep. Nevertheless, he had grown into a reasonably normal, gainfully employed adult, and he thought that the worst was behind him, until one night he awoke to find an intruder rummaging through his dresser drawers. After that, his nightmares began — terrifying, recurrent dreams in which the intruder was a middle-age woman and a knife dangled with Damoclesian contempt from the ceiling fan over his head.

“The old fear memories had not gone away,” said Dr. Ross Levin, a psychologist and sleep researcher at Yeshiva University in New York. They “were easily reactivated by the recent trauma,” and just as readily twisted into the basis of a repetitive nightmare. Dr. Levin urged the patient to reframe the dream and rehearse alternatives to swinging blades and frozen fear, until finally the nightmares abated and the man could regain his footing.

Few of us suffer from nightmares crippling and persistent enough to demand treatment. Yet we all know how bad a nightmare feels, how it surrounds you and surges up to drown you and makes your teeth fall out in chunks and gives you leukemia and look, your 6-year-old daughter is running back and forth through traffic, and oh no, this train is headed the wrong way and it’s past midnight, and there you are a cowardly third-grader back on Creston Avenue in the Bronx, no, please, not the Bronx! And you scream and you thrash and you want to wake up.

By all evidence, outrageously bad dreams are a universal human experience. Sometimes the dreams are scary enough to jolt the slumberer awake, in which case they meet the formal definition of nightmares — bad dreams that wake you up. At other times, they are even worse. The sleeper thinks the nightmare is over, only to step into Your Nested Nightmare, Chapter II. Whatever the particulars of the plot, researchers say, nightmares and dreadful dreams offer potentially telling clues into the larger mystery of why we dream in the first place, how our dreaming and waking lives may intersect and cross-infect each other, and, most baffling of all, how we manage to construct a virtual reality in our skull, a seemingly life-size, multidimensional, sensorily rich nocturnal roundhouse staffed with characters so persuasive you want to ... strangle them, before they can strangle you.