The calm before the panic attack.

We’ve all heard about the benefits of meditation ad nauseam. Those disciplined enough to practice regularly are rewarded with increased control over the brainwaves known as alpha rhythms, which leads to better focus and may help ease pain. In addition to calming the mind and body, meditation can also reduce the markers of stress in people with anxiety disorders. Rigorous studies have backed health claims such as these to convince therapists, physicians, and corporate gurus to embrace meditation’s potential.

What contemporary and ancient meditators have always known, however, is that while the hype may be warranted, the practice is not all peace, love, and blissful glimpses of unreality. Sitting zazen, gazing at their third eye, a person can encounter extremely unpleasant emotions and physical or mental disturbances.

Zen Buddhism has a word for the warped perceptions that can arise during meditation: makyo, which combines the Japanese words for “devil” and “objective world.” Philip Kapleau, the late American Zen master, once described confronting makyo as “a dredging and cleansing process that releases stressful experiences in deep layers of the mind.”

However, this demanding and sometimes intensely distressing side of meditation is rarely mentioned in scientific literature, says Jared Lindahl, a visiting professor of religious studies at Brown University, who has an interest in neuroscience and Buddhism. Along with Willoughby Britton, a psychologist and assistant professor of psychiatry at Brown, the two meditators have co-authored a study that documents and creates a taxonomy for the variant phenomenology of meditation. The paper, published in Plos One, is the beginning of an ongoing series of studies. “Just because something is positive and beneficial doesn’t mean we shouldn’t be aware of the broader range of possible effects it might have,” Lindahl says.

To conduct their research, the pair interviewed 60 Western Buddhist meditation practitioners who had all experienced challenging issues during their practice. They included both rookies and meditation teachers, many of whom had accumulated more than 10,000 hours of meditation experience in their lifetime. All belonged to either Theravāda, Zen, or Tibetan traditions.

The researchers identified 59 kinds of unexpected or unwanted experiences, which they classified into seven domains: cognitive, perceptual, affective (related to moods), somatic, conative (related to motivation), sense of self, and social. Among the experiences described to them were feelings of anxiety and fear, involuntary twitching, insomnia, a sense of complete detachment from one’s emotions, hypersensitivity to light or sound, distortion in time and space, nausea, hallucinations, irritability, and the re-experiencing of past traumas. The associated levels of distress and impairment ranged from “mild and transient to severe and lasting,” according to the study. Most would not imagine that these side-effects could be hiding behind the lotus-print curtains of your local meditation center.

However, the survey respondents didn’t necessarily perceive every non-euphoric event as negative. In fact, says Britton, she and Lindahl deliberately avoided the word “adverse” in their study for this reason. Instead, they chose “challenging,” which better captured the meditators’ varied interpretations of their experiences. For instance, a person who came away from a retreat feeling “very expanded and very unified with other people in the world” might have found their oneness with the universe distracting once they returned home. (That’s challenging, not tragic.)

The goal of the study was to look for patterns in the common accounts of unwanted reactions. Who runs into the unexpected hurdles? What are the unique set of factors involved? In which ways do teachers assist students who are struggling? (And do they blame inner demons for the upsets, or maybe something you ate at lunch?) The answers, which still require future research, may one day be relevant to the ways meditation is used as therapy.