We were once convinced that humans had only five senses: taste, touch, smell, sight, and hearing. Colloquially, we like to think we have a "sixth sense" that can tell us a variety of things, including when we're in danger, when someone likes us, or other ephemeral, unseen signals.
In fact, we have at least six senses, with some scientists saying we may have as many as 33 ways to perceive the world around us, within us, and beyond.
It's called interoception, and like our other senses, some people are better attuned to this sense than others, just like some can see better or hear better than most of us. Interoception is the body's ability to sense and interpret its own internal signals. This is what allows you to unconsciously or consciously feel your heart beating, recognize hunger, sense a full bladder, or feel your breathing. It is heavily tied to emotional regulation and mental well-being. In fact, being able to regulate your emotions, as many of us can do by sensing our heart and respiration and consciously slowing our breathing and thinking calming thoughts, can contribute to our overall mental health.
"Although we don't take much notice of it, it's an extremely important sense as it ensures that every system in the body is working optimally," said psychologists Jennifer Murphy of Royal Holloway University of London and Freya Prentice of University College London. They wrote an article in The Conversation in 2022. "It does this by alerting us to when our body may be out of balance, such as making us reach for a drink when we feel thirsty or telling us to take our jumper off when we're feeling too hot."
But researchers are now beginning to realize that interoception goes beyond simply regulating our biological needs, and may play a part in a range of mental health conditions – including anxiety, depression, PTSD, and eating disorders.
It's still early days, but the general idea is that our awareness of things such as our muscle tension, breathing and heart rate can give us important clues about when a situation is 'safe' or 'unsafe'.
When interrupted, this process could contribute to mental health conditions.
For example, someone with anxiety might be acutely aware of their heart rate in a situation such as a social interaction, which makes them feel uncomfortable in that situation.
An experiment published in eBioMedicine tried to measure how hunger impacted mood. The experiment showed "that people with strong and accurate interoception experienced fewer mood swings than those with poor interoception," according to Science Alert's Fiona Macdonald.
"This does not mean they never felt hungry – they just seemed better at keeping their mood levels stable," medical psychologist and corresponding author Nils Kroemer from the University of Tübingen in Germany wrote for The Conversation.
More tangible proof of interoception comes from a study of people suffering from anorexia nervosa by scientists at UCLA. The theory is that people with anorexia have stopped "listening" to their own internal hunger signals.
"People with anorexia nervosa do not simply ignore signals from the body," said Sahib Khalsa, the study's senior author and a neuroscientist at UCLA. "Rather, their nervous system may process gut sensations differently, making those signals harder to detect, trust, and learn from. Over time, that may contribute to the persistence of symptoms even after weight is restored."
"What we can say for sure is that humans are much more sensory than we give ourselves credit for. Even if we don't have a name for those senses as yet, they're already playing a bigger role in our wellbeing than we realize," writes Macdonald.
It's hoped that a better understanding of factors that affect interoceptive ability could someday lead to better treatments for people with many mental health conditions.
