It’s probably a rare thing for patients going into surgery to feel confident that they’ll come out unharmed. After all, they’re about to be cut up; their body invaded. Undoubtedly, topping the list of preoperative anxieties is the fear that they won’t be anesthetized enough, waking up mid-surgery, with their bodies open, unable to move and feeling everything. As it turns out, renditions of these incidents, called anesthesia awareness, occur in about 300 people per year in the UK and Ireland.

All of these patients were given general anesthesia, a cocktail of intravenous drugs with several purposes: They cause amnesia, pain relief, muscle paralysis, and sedation. Anesthesia awareness occurs when one of these drugs isn’t administered in as strong a dose as the others. Interested in seeing just how often it occurs, researchers from the UK’s Royal College of Anaesthetists and the Association of Anaesthetists looked into occurrences throughout the UK and Ireland, and their psychological effects on patients.

They found that one out of every 19,000 operations — other estimates put that number at one in 1,000 — resulted in an incidence of anesthesia awareness, and that 41 percent of patients who awoke mid-surgery were psychologically damaged for years after. “For the vast majority, it should be reassuring that patients report they were conscious during operations so infrequently. However, for a small number of patients, this can be a highly distressing experience,” said lead researcher Professor Tim Cook, of the Royal United Hospital, according to the BBC.

The study involved analyzing data on over three million operations that occurred during a single year. The researchers found that about one in 670 women undergoing C-sections with general anesthesia experienced some level of awareness, which was often a result of a miscalculation in the administration of sedation drugs; while the doctors wanted the patient unconscious, they wanted the baby to remain awake. Other times a person woke up included during heart and lung surgeries for obese people.

Fortunately, the majority of them only awoke for a brief period of time — either before surgery or once it was completed. However, that amount of time was just enough for them to feel their bodies paralyzed and unable to talk, causing them to panic, feel pain, or in some cases, choke.

For one patient who underwent orthodontic surgery (surgery of the facial bones) at the age of 12, waking up during the procedure caused 15 years of subsequent nightmares in which she said monsters leapt out of the darkness to paralyze her. “I could hear voices around me and I realized with horror that I had woken up in the middle of the operation but couldn’t move a muscle,” she told the BBC about her experience with anesthesia awareness. “While they fiddled, I frantically tried to decide whether I was about to die.” It wasn’t until she connected this experience to her nightmares that she was able to sleep peacefully.

According to the American Society of Anesthesiologists, anesthesiologists are required to monitor patients during the entire course of their surgery. During this time, they’re supposed to make sure patients are getting proper amounts of anesthetic drugs; that they’re breathing properly, with the help of oxygen and mechanical ventilators; that their blood — and the drugs — is circulating properly, and that body temperature is constant.