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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.

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