When you think of meditation you might think about highs of relaxation and calmness, but that's not always the case: a new study finds that up to a quarter of regular meditators have had bad experiences with the practice.

Anxiety, fear, and an altered sense of reality were some of the examples of negative meditation experiences researchers cited to 1,232 regular meditators, and 315 participants (25.6 percent) said they identified with them.

But the team behind the study says the aim is not to give meditation a bad name, just to highlight some of the ways it can occasionally go wrong – and work out ways to keep meditation a positive experience in the future.

"These findings point to the importance of widening the public and scientific understanding of meditation beyond that of a health-promoting technique," says lead researcher Marco Schlosser, from University College London in the UK.

"Very little is known about why, when, and how such meditation-related difficulties can occur: more research is now needed to understand the nature of these experiences.

"When are unpleasant experiences important elements of meditative development, and when are they merely negative effects to be avoided?"

Everyone interviewed for the study had at least two months' previous meditation experience, and were asked if they had ever had a "particularly unpleasant" moment while in a meditation session.

The researchers found that female survey respondents and those with religious beliefs were less likely to have negative meditation experiences.

On the flip side, those who had attended a meditation retreat, those with higher levels of negative thinking, and those who had only practiced deconstructive types of meditation (like Vipassanā or Kōan) were more likely to have negative experiences.

"Most research on meditation has focussed on its benefits, however, the range of meditative experiences studied by scientists needs to be expanded," says Schlosser. "It is important at this point not to draw premature conclusions about the potential negative effects of meditation."

It's worth considering the new study in the context of other meditation research. Scientists have previously found that meditation can boost the immune system, reduce stress levels, and even make us more aware of our unconscious mind.

That's a lot of positives, even if some negatives are also experienced by some people. The study authors note they didn't ask participants in detail about their bad experiences, and also didn't take into account existing mental health problems.

Future studies could go into more depth in those areas, the researchers say, deepening our understanding of how meditation sometimes doesn't have the intended effect. If experts can understand more about these potential problems, then we might be able to avoid them in the future.

The research could also inform related health practices as well, like therapy and counselling, and make them more suited to individuals.

"Longitudinal studies will help to learn when, for whom, and under what circumstances these unpleasant experiences arise, and whether they can have long-term effects," says Schlosser.

"This future research could inform clinical guidelines, mindfulness manuals, and meditation teacher training."

The research has been published in PLOS One.