[♪ INTRO ].

Being sick is the worst. Not only do you get the chills and a runny nose during the day, but your illness can chase you into your dreams as well.

You might have experienced those bizarre and emotionally intense dreams some people have when their temperature skyrockets,. AKA fever dreams. While lots of people seem to have these disturbing, illness-related nightmares, they haven’t received a lot of scientific attention.

But most experts think they’re an unfortunate side effect of a slightly heated brain. Survey studies suggest that fever dreams feature things like feelings of spatial distortion, falling into darkness, or nightmarish creatures. So yeah, they’re usually considered negative.

And back in 2013, a small study trying to characterize all the symptoms that go along with fevers was the first to suggest that, when it comes to fever dreams, what you’re sick with doesn’t matter. When they asked 28 subjects about their symptoms, classic things like shivering and sweating showed up, but three of them experienced weird enough dreams to get researchers interested. The subjects didn’t share a diagnosis; the fever seemed to be the only thing they had in common.

That suggests it’s not the particular ailment you have that messes with your dreams, but the fever itself. Which is something experts have long suspected because we know way more about how increases in temperature affect your brain. Regardless of whether it’s because of illness, heat stroke, or certain drugs, higher temperatures can increase the speed at which neurons transmit signals.

Which, in turn, increases their activity. And that can include neurons in the amygdala, the part of your brain responsible for handling fear and anxiety. Even in healthy people, the amygdala is activated during REM sleep, the part of the sleep cycle where you’re busy dreaming.

And more intense activity in the amygdala during sleepy time is associated with nightmares. So it’s possible that the increased heat from a fever ramps up the neurons in your amygdala, leading to those monstrous fever dreams. But a higher temperature’s likely not all you’re dealing with.

Your fever probably came from being sick, which means a very complex immune system response is also going on. When your immune system goes on the attack, little chemical signals called cytokines spread the call to arms all around the body, including to the brain. The amygdala has been shown to respond to these chemical signals by increasing its activity even in the absence of a big jump in temperature.

In rats and people, intentionally activating the immune system and triggering the release of cytokines increases anxiety. So during a fever, illness-induced chemicals and increased temperature likely work together, super-activating the amygdala to fill your head with all sorts of terrible things. And just when you had finally managed to get some rest.

Sweet dreams I guess? Thanks to Jonathan M for asking, and to all of our other patrons who voted for this question in our poll. If you want to pose questions like this or help decide what questions we answer, or even just receive some cool rewards you can’t get elsewhere, go to patreon.com/scishow. [♪ OUTRO ].