What is fever and why is it higher when it gets dark?

The clock reads 7 pm.

You've been dragging around that typical flu-like malaise all day. Your head aches like there's a DJ inside your skull testing the bass, you have chills for no reason, and your fever has been flirting with you, hovering around 37.5º all day. Annoying, but tolerable.

Until night begins to fall.

Suddenly, without warning, you feel something change. Your skin burns. Your temperature rises and rises as if your body has turned on an oven. You look at the thermometer: 38.6º. Excuse me? What kind of immune joke is this?

“You know that fevers always spike around this time,” someone tells you matter-of-factly, as if reminding you that bars start serving churros at six. And of course, you wonder: Why on earth is it always in the late afternoon or evening? What is it about darkness that makes our immune system go into overdrive? Does my body even know what time it is?

But before I tell you why fever spikes when the sun goes down, there's an even more important question…

What is fever and what purpose does it actually serve?

Fever isn't an enemy... it's a strategy. When your temperature rises, it's not because the virus is cooking you from the inside out. Viruses and bacteria don't have ovens or thermostats. It's your own body that, upon detecting the threat, decides to raise the temperature as if it were saying: "Okay, if you want war, you're going to get it... but in extreme mode."

Fever is a weapon against infections; it's not a bad thing for us. But having a fever is a perfect indicator that something is happening inside our body, and like any weapon, it can also turn against us. Hence the importance of fighting to control it.

Fever is a defense response. It's a command that originates in the hypothalamus, the part of the brain that controls your temperature. This command center has a set point, much like the temperature setting on your air conditioner. When a pathogen enters the scene, the immune system releases messenger molecules called pyrogens (such as interleukins IL-1, IL-6, and the well-known TNF-α). These messengers travel to the hypothalamus and tell it, "Hey, turn up the temperature, there are enemies here."

The hypothalamus obeys and activates a series of reactions to raise body temperature:

• Vasoconstriction: the blood vessels in the skin narrow so you lose less heat. That's why you feel cold and shiver when your fever starts to rise.
• Chills: your body contracts its muscles to generate extra heat. An involuntary immune response.
• Increased metabolism: your cells burn more energy, as if they were switching from energy-saving mode to turbo mode.
• Hormonal changes: hormones such as prostaglandin E2 are activated, which intensify and maintain this rise.

All of this causes your body temperature to rise from 36.5–37ºC to higher levels. And for what purpose?

Burn the enemy

The evolutionary logic is simple: many viruses and bacteria function less efficiently in hot weather. At feverish temperatures:

• The pathogens replicate more slowly.
• The cells of the immune system (lymphocytes, macrophages) become faster and more efficient.
• Antibody production is better activated.
• Cellular repair and cleaning mechanisms are working at full speed.

In other words, the fever is like increasing the difficulty of the game for the enemy, while you unlock defensive strategies.

And now that we know that fever is not a punishment, but a master plan of the immune system… it's time to answer the other big question:

If it's good, then why do I have to monitor it to make sure it doesn't rise too high?

A little fever is good… but only up to a point. Just like chocolate, beer, or scrolling through TikTok “for a bit.”

Your body raises its temperature to make things difficult for enemies, but if it goes too far, it starts to harm you too. Because, although the immune system is a genius, it's not exactly the king of moderation, and it will continue to raise its temperature as much as necessary to fight the infection it's battling.

Starting at around 39–40 °C, proteins and cells begin to become somewhat erratic: they become disordered, stop functioning properly, and your body enters dangerous territory, potentially leading to seizures and death.

So… what is the reasonable limit?

• Useful fever: 38–39 °C → the immune system celebrates, you sweat a little and you're still alive
• Very high fever: ≥ 40 °C → Serious problems begin, go to the hospital quickly if you cannot get it to go down.
• Above 41 °C → High risk of death

Above 40°C, proteins begin to deform and stop functioning properly, neurons become more sensitive (increasing the risk of seizures), and organs lose efficiency. This is the point at which fever ceases to be a defense mechanism and begins to pose a real problem for the body.

Above 41°C, the brain enters the red zone: the risk of severe seizures increases, enzymes stop functioning, tissues can begin to be damaged, and the heart has to work at its limit. At this point, fever is no longer a defense mechanism… it's an internal fire that must be extinguished with urgent medical help.

Furthermore, the most vulnerable people (young babies, the elderly, people with chronic illnesses, pregnant women…) cannot afford such high temperatures, because their bodies have less room to play superheroes.

What can actually kill you with an extreme fever?

When the temperature rises above 41–42°C, the body can no longer control the heat. Enzymes and proteins begin to deform, so cells stop functioning. The brain is the first to fail: seizures, edema, and severe neurological failure occur. And if the heat continues to rise, the heart and vital organs run out of energy, as the heart becomes exhausted and blood pressure plummets, leading to a cascade of collapse known as multiple organ failure.

The infection doesn't kill you... your own out-of-control inner fire kills you.

And why are babies and children so explosive with a fever?

Because its temperature control system is like a beta version app: it works... but sometimes it crashes.

In adults, the hypothalamus is already a seasoned engineer, able to regulate temperature with considerable precision. In children, however:

• The hypothalamus is still maturing → it regulates with less finesse
• Their body surface area is greater in proportion to their size → they gain and lose heat faster
• Its immune system reacts dramatically → when it detects a virus, it cranks up the temperature to the max

Result: their fever rises faster and higher.

And if the temperature jump is very sudden, something very frightening can happen: febrile seizures.

These seizures are usually brief and leave no lasting effects, but of course, seeing your little one have a seizure is one of those experiences that can make your life feel like it's been 5 years ago out of sheer fright.

Therefore, in children:

• The fever needs to be monitored more closely.
• Lower the temperature if it spikes or rises rapidly.
• And always consult a doctor if there are seizures, uncontrolled fever, or if they are very young (less than 3–6 months old).

So why does the temperature rise in the late afternoon and evening?

First, we need to talk about something your body does every day without asking permission : following a circadian rhythm. It's an internal clock that decides what time you should wake up, when you get hungry, when you should sleep… and how your immune system behaves throughout the day.

The great conductor of this orchestra is the hypothalamus, and one of its favorite tools is called cortisol: a hormone we've already discussed at length in Science Driven that is high in the morning (activating you, putting you in "productive mode") and gradually decreases throughout the day until reaching minimal levels at night, allowing sleep to be induced. This is known as the cortisol cycle.

And what does cortisol have to do with fever? A lot.

Day: Survival

With high cortisol:

• Inflammation is slowed
• The immune system is calm
• Priority: that you stay functional, move, work, don't collapse

Your body thinks:

“We have to survive the day, we can’t be fighting like crazy while you’re trying to live your life.”

Night: War Mode

When cortisol levels drop:

• More inflammatory cytokines are released
• Pyrogens are more active
• The internal thermostat raises the set point
• The immune system says: “Now it’s time to attack!”

Why at night?

Because when you sleep, you're not distracting the system with walking, thinking, eating, endless scrolling… It's the perfect time to invest energy in repairing and destroying pathogens without interruption.

It's not that the virus said, "Let's cause more trouble at night." It's your immune system choosing its best time to fight. Your body knows exactly what time it is... and takes advantage of the darkness to become an inflammatory ninja.

So yes: when you're trying to sleep, the fever takes advantage to work overtime.

How do fever-reducing medications work?

Now that we know that fever is a strategy of the body, it's time to talk about plan B: how to help your body when it gets too intense.

Medications that lower fever are called antipyretics, and their trick is quite elegant: they block the chemical signals that tell the hypothalamus to raise the temperature. Specifically, they slow the production of prostaglandins, the molecules that keep fever high. If prostaglandins decrease, the brain's thermostat lowers, and your body stops heating up.

The two kings of the party: paracetamol and ibuprofen

Medicine	How it works	When is it convenient
Paracetamol	It lowers fever and relieves pain by acting on the nervous system.	Fever with headache, in children, when there is no inflammation
Ibuprofen	In addition to being an antipyretic, it is an anti-inflammatory (NSAID).	There is visible inflammation: swollen throat, very sore muscles

 

Which one is better?

There is no single "absolute champion": it depends on your situation.

• In young babies, paracetamol is usually preferred.
• In adults with pain and inflammation, ibuprofen is preferred. Sometimes, both are even alternated (common in children).

Never take them "just in case": fever is useful as long as it doesn't spike.

Other antipyretics… less classic

There are also medications such as naproxen, aspirin, ketoprofen, metamizole… but they are not the first choice for common fever, especially in children (aspirin, for example, can cause Reye's syndrome, which is a very serious problem).

And what can I do at home when my fever rises?

Besides medication, there are very simple tactics to help the body without fighting against its strategic plan:

• Stay well hydrated; you lose water easily when you have a fever.
• Cool weather, no need to bundle up like you're going to Mount Everest
• Warm showers (32º - 36º) to lower the temperature and improve discomfort
• Light clothing and thin sheets
• Warm (not ice-cold) compresses
• Rest, your immune system needs energy
• Temperature checks at regular intervals

Avoid cold showers (they cause vasoconstriction and can raise fever), excessive clothing or blankets, and self-medicating with antibiotics just because; they don't lower fever, they only work for bacterial infections, and each bacteria has a specific antibiotic, you won't get it right.

When do you absolutely have to go to the doctor?

• Fever > 40 ºC or that does not decrease with antipyretics
• Seizures, confusion, or fainting
• Difficulty breathing
• Signs of severe dehydration
• Babies < 3 months with fever (direct emergency)
• If it lasts more than 3 days without explanation

Conclusion

Fever, in reality, is a symptom that warns us and a mechanism that protects us; we must let it work but without it becoming the villain of the story.

Our body is smarter than it seems… we just have to help it not to burn itself out in the attempt.