Why does a fever seem to spike at night?

April 30, 2018

Elizabeth Meade, MD

young girl lying in bed with thermometer in her mouth

[3 MIN READ]

Health care professionals have known this about illness for many years, anecdotally. But science is beginning to offer some explanation and validation for the theory. There’s even a field of study devoted to understanding how the time of day affects our health — chronobiology. Let's take a deeper look at how fevers work and why they seem worse at night.

What is fever?

The primary symptom of fever is abnormally high body temperature.

Normal body temperature for an adult can range anywhere from 97 F to 99 degrees F. Children have a slightly higher range, approximately 98 to 100 F. However, adults are much more tolerant of temperature spikes and have more fully functional immune systems than children. With children — especially babies — even a slightly elevated temperature can signal something more serious.

In addition to a high temperature, symptoms of fever can include shivering and sweating, headache, muscle ache, loss of appetite and general fatigue. In some cases, children under five may suffer seizures during high fever spikes — alarming for parents, but not usually life-threatening.

It’s important to remember that fever itself is not a disease. In fact, it’s a sign that the body’s immune system is fighting off a bacterial or viral infection — although it can be a cause for serious concern in certain circumstances. For example, urgent medical evaluation is needed if an infant less than two months of age is running a fever greater than 100.4 F, or if anyone with a compromised immune system spikes a fever.

Is “night fever” really worse?

“Night fever” refers to the tendency of fever symptoms to worsen during the evening hours, and there are some clear explanations for why it happens.

It’s normal for body temperature to creep up at night, adding proverbial fuel to the fire. This is often compounded by well-meaning parents or caregivers swaddling their infant in piles of blankets. It’s because of this natural ebb and flow of body temperature that health care professionals don’t declare a fever broken in the morning just because someone suddenly seems symptom-free. Instead, they often wait until the fever has been absent for 24 hours.

There’s also the undeniable effect of boredom. During the day — even if you have a fever — you tend to be more active or at least distracted by visitors, making sure you’re drinking lots of fluids and getting enough rest. At night, it’s just you, the four walls, and your fever — so you have more time to think about how miserable you feel. And that makes you feel more miserable.

But it’s not all perception — fevers actually can be worse at night. The inflammatory response mechanism of the immune system is amplified. Your immune system deliberately raises your body temperature as part of its strategy to kill the virus attacking you. That’s what triggers the alternating hot flashes and chills — the classic fever effect.

There’s one other element that medicine doesn't fully understand. We know that two key hormones – cortisol and adrenaline – are suppressed when we sleep. From extensive studies on asthma management, we have learned that when the level of these hormones is reduced at night, it’s harder for asthmatics to breathe. Researchers believe this restriction also exacerbates fever symptoms at night.

When you or your family members have a troubling fever, trust your instincts if you think something is wrong and call your pediatrician or family doctor for advice. And remember, always call a doctor if a young infant has a fever of more than 100.4 F.

Elizabeth Meade, M.D., is Swedish’s medical director of education, outreach and quality, pediatrics.

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