[Editor’s note: This article was originally hosted on MyFamilyDoctorMag.com, our sister site.
It’s now featured here as part of our new general-health section.]

by Emmanuel Rodriguez, M.D., M.P.H.

Q. Why does a fever sometimes get higher at night?

A. The answer to that is pretty simple: Body temperature, whether you’re sick or well, just gets higher later in the day. But the explanation behind that answer has to do with all sorts of things.

Three factors regulate your body temperature:

  1. A small gland at the base of the brain called the hypothalamus
  2. Your body’s vital functions (such as your muscle activity and heart rate)
  3. The temperature of your surroundings
Fever Cause: Your Inner Thermostat

The hypothalamus is your body’s built-in thermostat. By secreting hormones in small pulses, it communicates with your other vital organs, carefully regulating your body temperature close to a set temperature of 98.6 degrees Fahrenheit.

If you produce excess heat by exercising, your hypothalamus reduces your temperature by increasing the blood flow to your skin (basically, drawing more heat out of your body) and by making you sweat. On the other hand, if you go out into the cold, your hypothalamus tries to increase your temperature by causing you to shiver.

When you have a fever, the hypothalamus has reset your body temperature to one that’s higher than normal. It does this for an unclear reason. Some people think fever helps the body’s immune system fight the infection because some bacteria don’t thrive on higher temperatures.

Home remedies + science = do-it-yourself survival medicine! Get prepared for disasters with The Survival Doctor’s e-books.

Why a Fever Is Higher at Night: Your Heat Cycle

Books adYour temperature usually follows a built-in 24-hour cycle. Its lowest point is between 3 and 6 a.m., followed by a peak between 4 and 11 p.m.

Two major factors regulate this cycle:

  1. Your hypothalamus has its own 24-hour hormone-secretion pattern. We don’t know the reason for this so-called circadian rhythm, but to some extent the day/night light cycle helps regulate it.
  2. The things the body does during the day (heartbeat, muscle movements, breathing) involve a release of heat energy, causing your core body temperature to warm up as the day progresses.

This explains why your temperature increases toward the end of the day under normal conditions. However, this cycle still happens when you have a fever. The difference is that now, the temperature elevation is more obvious since you’re already starting from a higher temperature than normal.

There are exceptions to this cycle. Outside factors that can dampen the evening temperature-elevation include older age; certain medical conditions, such as diabetes; and the use of some common drugs, such as acetaminophen (Tylenol) or aspirin—all of which affect the functioning of the hypothalamus.

Board certified in internal medicine,
 EMMANUEL RODRIGUEZ, M.D., M.P.H., is an infectious-disease specialist with NorthReach Internal Medicine Clinic in Marienette, Wis., and attending physician and hospital epidemiologist with Bay Area Medical Center.

(Subscribe to updates below.)