Now that we've got a handle on fevers and dreams, let's put them together. What is it about fevers that makes our dreams so horrible? If, under normal circumstances, our dreams are supposed to help us neutralize difficult emotions, why do they terrify us when we're sick?
Remember that dreams occur in your brain, and your brain is quite different from the rest of your body. While it makes up just 2 percent of your total body mass, it needs 20 percent of the oxygen you breathe. Plus, neurons, the main component of the brain, are high-wattage cells; they require a lot of power to get the job done. In fact, a single neuron needs anywhere between 300 and 2,500 times more energy than the average body cell [source: Kiyatkin].
All that energy dissipates as heat, and to run well, the brain, like a computer, must be careful not to get too hot. That can be a challenge since the temperature in your gray matter tends to go up higher and faster than the rest your body when dealing with any kind of challenge (like a fever). Certain recreational drugs (e.g., meth, ecstasy and MDMA) are known to raise brain temperatures, which is one of the main reasons they can be toxic to your neurons [source: Kiyatkin].
We know that all forms of mental activity are highly sensitive to temperature changes. One study even showed that when we're exposed to higher temperatures our ability to make smart decisions decreases significantly [source: Ward]. That doesn't mean that people in hot climates make bad decisions — given enough time, our bodies can adjust to higher or lower temperatures. But an unusually hot day could be a bad time to choose a data plan or bet on the ponies.
As discussed earlier, our bodies are remarkably adept at controlling our internal temperatures. But that's not always true. It turns out that during REM, your interior thermometer tends to develop technical difficulties [source: Science Focus]. Add a fever to that and throw in the fact that your brain's temperature spikes faster and higher when the body is challenged, and then mix in the adverse effects of heat on brain function, and you've got a recipe for disastrous dreaming.
Remember the amygdala, that part of the brain closely linked both to dreams and the processing of intense emotions? It's particularly associated with negative emotions like terror and anger. Specialists think the amygdala is the culprit behind your garden variety nightmare [source: McNamara]. It would seem that it goes especially haywire under the fever-plus-REM scenario and just starts shooting out horrific dream matter willy-nilly like some misfiring grenade launcher. The result: fever dreams.