A fever signals an elevated body temperature beyond the typical range. Normal body temperature can fluctuate throughout the day, often being slightly lower in the morning and higher in the evening. While these daily variations are natural, a fever can feel particularly pronounced or worsen during nighttime hours. This often raises questions about its underlying causes.
Your Body’s Natural Temperature Cycle
The human body operates on an internal biological clock known as the circadian rhythm, which influences physiological processes like core body temperature. This rhythm dictates that body temperature is lowest in the early morning hours and gradually rises throughout the day, peaking in the late afternoon or evening. Even in healthy individuals, this natural cycle causes body temperature to be slightly elevated during the night.
Hormones, like melatonin and cortisol, play a role in regulating this circadian fluctuation. Melatonin, which promotes sleep, is produced as evening approaches, while cortisol, associated with wakefulness, decreases. This interplay of hormones and metabolic activity contributes to the natural temperature rise at night. This nocturnal increase can make any concurrent fever, even a mild one, feel more intense or noticeable when resting.
Underlying Illnesses and Infections
A primary reason for elevated body temperature, especially at night, involves the immune system’s response to pathogens. When the body encounters viruses, bacteria, or other infectious agents, immune cells release substances called pyrogens. These pyrogens signal the hypothalamus, the brain’s thermoregulatory center. The hypothalamus then resets the body’s temperature set point to a higher level, initiating a fever.
The immune system is more active during nighttime hours, which can lead to a more pronounced fever response. This heightened nocturnal immune activity may contribute to fevers feeling worse at night. Common infections that frequently cause fevers include the common cold, influenza, bronchitis, urinary tract infections, and pneumonia. In such cases, the fever is a sign that the body is actively fighting off the infection.
Less Obvious Contributors
Beyond typical infections, several less common factors can contribute to nocturnal fevers. Certain medications can induce a drug-induced fever as a side effect. These can include some antibiotics, anticonvulsants, and some blood pressure drugs. The fever resolves within a few days of discontinuing the medication.
Inflammatory conditions, like autoimmune diseases such as rheumatoid arthritis or lupus, can also manifest with chronic or intermittent fevers. Systemic inflammation associated with these conditions can trigger the body’s fever response. Hormonal changes, particularly during perimenopause and menopause, can lead to temperature fluctuations and fever due to declining estrogen levels. In rarer instances, certain underlying medical conditions, including some cancers like lymphomas and leukemias, can cause fevers at night, often accompanied by other symptoms like unexplained weight loss or night sweats.
When to Consult a Doctor
While many fevers resolve on their own, there are situations requiring medical attention. Consult a healthcare provider if a fever reaches 103°F (39.4°C) or higher. Also, seek medical advice if a fever lasts longer than three days, even if it is not particularly high.
Seek immediate medical attention if a fever is accompanied by severe symptoms like difficulty breathing, a severe headache, a stiff neck, confusion, a new rash, or severe pain. Fevers in infants younger than three months require medical evaluation, especially if the rectal temperature is 100.4°F (38°C) or higher. Individuals with compromised immune systems or chronic health conditions require prompt medical consultation for any fever, due to higher risk of serious infections.