Thanks to Brilliant for supporting this episode of SciShow!

Go to Brilliant.org/SciShow to learn more. {♫Intro♫}. Being sick sucks.

But you might have noticed it often sucks more at night. Maybe you can push through the day feeling just a little toasty and gross, but after sunset, you're on a one-way bus to Fever Town. And that wasn't your imagination.

Fevers do often rise at night, and that's largely because that's when our bodies naturally stoke their internal furnaces. Your body temperature is controlled by your hypothalamus, a small region at the base of your brain. It can sense the temperature of the blood that passes through it, and receives temperature information from your skin, too.

And it uses all this info, as well as chemical signals from your body, to calibrate your internal temperature to a cozy 37 degrees C. But even when you're totally healthy, it doesn't keep your body temperature completely stable throughout the day. Instead, you fluctuate by up to half a degree Celsius in either direction thanks to your body's circadian rhythm — essentially your body's internal clock.

That rhythm is controlled by a tiny region of the hypothalamus called the Suprachiasmatic Nucleus, or SCN for short, which receives light and dark signals from nerve cells in your eyes as well as input from other parts of your brain. Usually, these rhythms cause our body's temperature to dip to its lowest point around 4 in the morning, and then to the high end of the normal range around 6 at night. And this daily cycle doesn't stop when you're sick.

So during fever, not only is your temperature elevated, it's still subject to that upward swing in the evening. And that's not all. Circadian rhythms also influence your immune system.

You see, fevers are triggered by substances called pyrogens. These can come from a few places. White blood cells can release them into the bloodstream when they sense an intruder.

Or, might be emitted more directly by infected tissue or the pathogens themselves. But wherever they're from, the effect is the same: they tell the hypothalamus to ramp your thermostat up. And because of that, the daily cycles of your immune system could add to your nightly temperature spike.

You literally have more white blood cells at night, for example, and studies have found some pyrogen levels also tend to spike in the evening. Now, there are exceptions to this nightly fever pattern, of course. Bacterial pneumonia and typhoid fever, for instance, typically cause fevers that stay elevated all day and night without that daily fluctuation.

And some kinds of malaria lead to fever spikes that occur on 2 or 3 day cycles instead of daily. So keeping track of how your temperature changes over time could help your doctor figure out what's got you feeling so crummy. And in general, you should probably check your temperature at the same time every day if you really want to know if you're getting better or worse.

The best time is probably somewhere between 6 and 8 pm, since that's when your temp will likely be the highest. Of course, I am not a doctor, so if you have any concerns about a fever at any time of day, you should definitely consult a healthcare professional. And if you do find yourself stuck at home thanks to a particularly nasty bug, you might as well do something fun with your downtime.

Like, take a course or two from Brilliant.org! You see, learning doesn't have to be dull. Brilliant offers a variety of engaging courses in math, science, and computer science that are designed for ambitious, curious people.

For example, their puzzle science course teaches physics the fun way by getting you to solve puzzles about mirror reflections, laser tag, and making the perfect shot in a game of pool. Right now, the first 200 people to sign up at Brilliant.org/SciShow will get 20% off the annual Premium subscription. And if you do, you'll also be supporting SciShow—so thanks! {♫Outro♫}.