What is the circadian rhythm?
The human body has a built-in 24-hour "body clock", called the circadian rhythm in medicine. This mechanism governs many physiological functions, from the sleep–wake cycle, hormone release and metabolism to the regulation of body temperature.
A normal person's body temperature does not stay the same all day. It rises and falls in a regular pattern at different times of the day:
In other words, the same reading of 37.8°C may already count as a clear fever if it is taken early in the morning, but may be only a normal high reading if it is taken in the early evening. When healthcare staff assess a fever, they must therefore always consider the time at which the temperature was taken.
What a fever in the early morning and in the early evening shows about the body
When the body has an infection or inflammation, the course of the fever often "magnifies" the body's natural daily temperature rhythm:
Normal temperature compared with fever
- In a healthy person, body temperature varies by about 0.5°C within a day.
- When the body has an infection, the difference between day and night temperature may widen to 1–2°C.
Fever in the early morning
- Body temperature should be at its lowest in the early morning, yet there is still a fever.
- This shows that the body's temperature regulation is abnormal.
Fever in the early evening
- In the early evening, when body temperature is already at its highest, the fever reaches a higher temperature still.
- This may mean the infection in the body is more serious.
Loss of the rhythm
- If the daily temperature rhythm disappears completely and a high fever lasts all day.
- This may be a warning sign of severe infection.
Taking and recording the temperature at set times (every 4–6 hours is suggested) helps to show the course of the fever and is very useful to healthcare staff in judging the condition. When recording, it is best to write down both the time of the reading and the method used.
Common patterns of fever
Different illnesses show particular patterns of fever, and noticing these ups and downs can sometimes help to work out the cause:
Continuous fever
- Body temperature stays at a high level and does not come down.
- Within 24 hours, the temperature varies by less than 1°C.
- This is common in typhoid and pneumonia.
Remittent fever
- Body temperature varies by more than 1°C
- The temperature rises and falls, but does not come down to the normal range in between.
- This is common in abscesses and tuberculosis.
Intermittent fever
- High fever and normal temperature alternate.
- It comes once a day or every other day.
- This is common in malaria and urinary tract infection.
Relapsing fever
- After a fever for several days, the temperature returns to normal for several days.
- Fever and recovery alternate in cycles.
- This is common in certain infections.
What to pay special attention to when an older adult has a fever
The temperature regulation of older adults is different from that of younger people, and care must be taken accordingly:
A lower baseline temperature
- An older adult's normal temperature is usually lower and may be only between 36.0 and 36.5°C.
- A temperature of 37.2°C may therefore already be a clear fever for an older adult.
- The traditional fever standard for adults in general may not apply to older adults.
A weaker fever response
- Even with a serious infection, an older adult may not have a clear high fever.
- Body temperature may rise by only about 1°C above normal.
- Carers must also watch for other abnormal clinical signs in the older adult.
Other warning signs
- For example a sudden change in alertness, or becoming confused.
- A clear loss of appetite
- Or a sudden fall, or less mobility than usual.
For older adults who are bedbound for long periods or live alone, arranging regular home nursing visitscan help to establish the individual's baseline temperature. This helps to pick up early signs of infection and, when needed, to arrange medical care.
Practical advice for carers
- Use the same way of measuring (oral, ear or armpit) and take the temperature at similar times each day so that readings can be compared.
- Record the temperature reading, the time and method of measurement in detail, together with accompanying symptoms and any medicines taken.
- Compare the temperature at the same time of day, and look at the pattern of the fever and not a single reading.
- If the temperature goes above 39°C or the fever lasts more than 3 days, or is accompanied by a change in consciousness or difficulty breathing, seek medical care at once.
Are you looking after a family member with a chronic illness or limited mobility? Our home nurses can help to monitor temperature, medication and daily care.
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