It was only the third awakening that left me haunted by flashbacks and nightmares. What had gone wrong?

I soon learned that I am part of only a tiny percentage of people who remember unsettling experiences under conscious sedation. Only three out of every 10,000 people report “undesired awareness” from nongeneral anesthesia, a number only slightly higher than the two out of every 10,000 patients who report this under general anesthesia, according to a study led by Dr. George Mashour, a neuroanesthesiologist at the University of Michigan and one of the world’s experts on anesthesia awareness. While some patients expect, or even want, to be awake during certain procedures, especially colonoscopies, “I don’t think any clinician would want somebody to be terrified or in pain,” Dr. Mashour said.

But it happens.

His research, using the American Society of Anesthesiologists’ Anesthesia Awareness Registry, a voluntary registry of patients with memories under conscious sedation or general anesthesia, showed that 78 percent of those reporting awareness under conscious sedation felt distress, and 40 percent had long-term psychological sequelae, including post-traumatic stress disorder.

There has been extensive research and debate on how to prevent these awakenings, known as “anesthesia awareness,” under general anesthesia. But far less is known about their impact on people undergoing conscious sedation, which is growing in popularity and estimated to account for half of all anesthesia administrations in the United States within 10 years.

Often requiring no anesthesiologist, no operating room and not even a hospital setting, conscious sedation is now being used increasingly for dental work, plastic surgery, heart rhythm corrections and many more procedures performed by a growing range of practitioners, including certified anesthesiologist assistants and nurse anesthetists.

Part of the problem is that there is no firm definition of conscious sedation, which involves a cocktail of pain control, anxiety control and amnesia in amounts that can vary between patients, procedures and practitioners, says Dr. Andrew Davidson, head of anesthesia research at the Murdoch Children’s Research Institute in Melbourne, Australia.

“Conscious sedation is a bit of an art,” Dr. Davidson said. “Some people say it’s actually harder to do than general anesthetic, because it’s actually more difficult to titrate the drugs to get exactly what you want.”