Nearly everyone has experienced a fever and its attendant symptoms—from fatigue and body aches to hot flashes and chills. You may also have noticed strange or unusually vivid dreams while trying to sleep off an illness. The phrase “fever dream” turns up in literature and popular culture, and while the scientific literature on fevers is surprisingly limited, it has documented the strange dream phenomenon.
In one small 2013 study that surveyed people’s fever symptoms, several individuals brought up “odd” dreams. One person mentioned a dream that recurred during their adolescence and returned during a recent fever. Another described dreams that seemed to shift repeatedly between pleasant situations and troubling ones.
A 2016 study documented the types of dreams some people had during a past fever, and then compared these to dreams people had when well. “Fever dreams are more bizarre, more negatively toned, [and] include less social interaction” than normal dreams, says Michael Schredl, one of the study authors and a professor of sleep research and psychiatry at Germany’s Central Institute of Mental Health.
Roughly 94% of the people who had experienced fever dreams described them as negative, his study found. They also rated these dreams as much more “emotionally intense” than their regular dreams. People in the study described “creatures with oversized arms and legs,” giant insects, and blackness “slowly spreading all over.”
Why would a fever cause odd and intense dreams? Elevated brain temperatures could disrupt the brain’s normal cognitive processes, Schredl and his coauthors write in the paper. “The idea is that the brain is not functioning well during high fever,” he says, and this somehow produces dreams with unusual and unpleasant qualities.
Sleep researchers have learned that most dreaming seems to take place during the rapid eye movement (REM) stage of the sleep cycle. “REM sleep is essential to temperature control, [and] fever suppresses REM,” says Dr. J. Allan Hobson, a sleep researcher and emeritus professor of psychiatry at Harvard Medical School. It makes sense that, if a fever disrupts REM sleep, it could also change the nature of a person’s dreams.
Schredl also points out that, in his study, illness symptoms and related imagery were much more likely to turn up in fever dreams than in normal ones. Some people in his study recalled dreaming of burning clouds and melting statues, and he says this could have been influenced by their brain’s awareness of fever-induced heat.
This lends credence to what he and his study coauthors call the “continuity hypothesis” of dreaming, which holds that our dreams often mirror recent aspects of our waking life. Looked at this way, it’s no wonder that—after spending a day at home in bed, struggling with a fever—a person’s dreams may be unpleasant and devoid of social interaction.
Much about dreams, including their meaning and purpose, remains veiled in mystery. And, somewhat surprisingly, given how common fevers are, “there’s a real lack of information on symptoms of fever,” says Nancy Ames, a nurse researcher at the National Institutes of Health Clinical Center who co-authored the 2013 study. No surprise, then, that there are still many more questions than answers when it comes to fever dreams.