A part of your brain called the hypothalamus controls your body temperature. This usually varies slightly throughout the day. A high temperature is called a fever (or sometimes a high fever). Fever itself is not an illness. A fever is usually a symptom of an underlying condition, most often an infection. Night sweats occur when you sweat too much at night. They can be related to a fever but they can also occur without a fever.
Your body temperature varies slightly throughout the day. A high temperature is also known as a fever. Generally, a fever is a rise of body temperature above the normal daily variation. Normal temperature varies depending on the person, the body site where temperature is measured, and the time of day.
The actual temperature causing a fever varies between different people and depends on what their usual temperature is. It is usually agreed that a person with a temperature of 38°C or higher has a fever. However, a temperature over 37.5°C can often be considered to be a fever in many children. Fevers can occur in children and adults and can be very common.
What is the body temperature?
Your normal body temperature changes throughout the day. These changes can be caused by exercise, eating, sleeping and even the time of day. Your temperature is usually highest in the early evening and lowest in the early hours of the morning.
Your average body temperature, taken with a thermometer in your mouth, is 37°C, but anywhere between 36.5°C and 37.2°C is often considered normal. Armpit temperatures are 0.2°C to 0.3°C lower than this.
How is body temperature taken?
There are different ways of taking your temperature. The quickest and easiest way of taking a temperature is with a thermometer. This can be by an electronic or chemical dot thermometer. A thermometer can be placed either under your armpit or in your ear. The forehead thermometers are no longer recommended.
As a parent it can be extremely worrying if your child has a high temperature (fever). However, having a fever is very common and often clears up by itself without treatment.
How common is a fever?
A high temperature (fever) can be really common. They are more common in children. Around 3 out of 10 young children have a fever every year and a fever is one of the most common reasons for a child to be seen by their GP.
What causes fever?
High temperature (fever) is caused by the release of certain chemicals by your immune system, usually as a result of infection or inflammation. Fever is an important sign that a person is unwell and a cause should usually be found.
Most fevers are caused by infections or other illnesses. Viral infections are very common causes of a fever. The high body temperature makes it more difficult for the germs (bacteria or viruses) which cause infections to survive.
Fever caused by infections
Common conditions that can cause fevers include:
- Respiratory tract infections and colds: see separate leaflet called Common Cold (and Other Upper Respiratory Tract Infections).
- Flu (influenza): see separate leaflet called Influenza and Flu-like Illness.
- Infection of the gut (gastroenteritis): see separate leaflets called Gastroenteritis in Children and Gastroenteritis in Adults.
- Ear infections: see separate leaflet called Ear Infection (Otitis Media).
- Infection of the tonsils: see separate leaflet called Tonsillitis.
- Kidney or urinary tract infections: see separate leaflets called Urine Infection in Children, Cystitis (Urine Infection) in Women, Urine Infection in Pregnancy, Urine Infection in Men, Kidney Infection (Pyelonephritis) and The Kidneys and Urinary Tract.
- Common childhood illnesses: for example, chickenpox (varicella), scarlet fever, rubella and whooping cough (pertussis). See separate leaflets called Chickenpox in Children under 12, Scarlet Fever, Rubella (German Measles) and Whooping Cough.
If you have been abroad and develop a fever, it is important to see a doctor. Infections sometimes caught abroad can also cause a fever. See separate leaflets called Malaria, Tuberculosis, Hepatitis A, Hepatitis B and Lyme Disease.
Your child's temperature can also be raised when their teeth start to develop (their teething period), following some vaccinations, or if they overheat because of too much bedding or clothing. See separate leaflets called Teething, Immunisation - Usual UK Schedule and Reducing the Risk of Cot Death.
Fever caused by other conditions
Less commonly, fever can be caused by conditions other than infections. For example, blood clots in the leg or lung can sometimes lead to a fever developing. See separate leaflets called Deep Vein Thrombosis and Pulmonary Embolism.
There are some conditions which cause swelling (inflammation) in the body. If you have one of these conditions, a common symptom you may experience is fever. See separate leaflets called Rheumatoid Arthritis and Systemic Lupus Erythematosus.
There are also some types of cancer which can lead to a fever developing. See separate leaflet called What is Cancer?
When should you seek urgent medical advice?
You should contact a doctor if you have a child with a high temperature (fever) and:
- Your child has a fit.
- Your child develops a rash that does not disappear when you press a glass on it (a non-blanching rash).
- You feel that your child is becoming more unwell.
- The fever lasts longer than five days.
- You are worried or concerned about looking after your child.
You should contact a doctor if you have a fever and have worsening symptoms - for example, a rash, stiff neck, shortness of breath or chest pains.
How is a fever treated?
A high temperature (fever) is usually associated with physical discomfort and most people feel better when a fever is treated. However, depending on your age, physical condition and the underlying cause of your fever, you may or may not require medical treatment for the fever alone. Many experts believe that fever is a natural bodily defense against infection.
Your doctor will examine you and try to determine the underlying cause for your fever. If it is due to infection with a germ (a bacterial infection) - for example, a urine infection - you may be given antibiotics. You may need to have some tests, especially if a cause other than an infection is likely. Testing may include:
- Blood tests.
- Urine tests.
- X-rays to determine the cause, especially where a cause other than an infection is a possibility.
Warm (tepid) sponging is no longer recommended for children with a fever. Drinking plenty of fluids is important when you (or your child) have a fever. Keeping fluid levels up is important to reduce the risk of lack of fluid in the body (dehydration).
Paracetamol or ibuprofen may help to reduce the fever. They are usually only recommended if your child is distressed with the fever.
Note: giving paracetamol or ibuprofen does not reduce the risk of a seizure caused by a fever occurring. See separate leaflet called Febrile Seizure (Febrile Convulsion).
What is a night sweat?
A night sweat is excess sweating at night. Although night sweats can occur if your bedroom is very hot or you have too many bedclothes on overnight, true night sweats occur at night. They can drench your nightclothes and are not related to being too warm in bed.
What causes night sweats?
There are many different causes of night sweats. Some of the conditions that can cause night sweats include:
Night sweats are often a very common symptom many women experience during their menopause. This is related to not having enough oestrogen in the body. If the night sweats are troublesome and you have other symptoms, taking hormone replacement therapy (HRT) can really help to improve your menopausal symptoms. See separate leaflet called Menopause and HRT.
Some tablets and medicines can cause night sweats as a side-effect. For example some anti-depressants and aspirin can cause night sweats. See separate leaflets called SSRI Antidepressants and Aspirin and Other Antiplatelet Medicines.
If your night sweats are due to to your medication it may be possible to take alternative medication. Your doctor will be able to discuss this with you.
Low blood sugar levels
Sometimes low blood glucose can cause sweating. If you are taking insulin or some types of oral diabetes medications such as sulfonylureas (for example, glipizide) then you may experience low sugar levels at night accompanied by sweating. See separate leaflet called Dealing with Hypoglycaemia (Low Blood Sugar).
Most infections can cause a fever with some sweating; however, night sweats are more common with certain types of infections. Tuberculosis (TB) is the infection most commonly associated with night sweats. See separate leaflet called Tuberculosis.
However, infections caused by germs (bacterial infections), such as inflammation of the heart valves (infective endocarditis), inflammation within your bones (osteomyelitis), and abscesses all may result in night sweats. See separate leaflets called Infective Endocarditis, Osteomyelitis and Abscess.
Night sweats can also be a symptom of HIV infection. See separate leaflet called HIV and AIDS.
Night sweats can sometimes be an early symptom of some cancers. The most common type of cancer associated with night sweats is lymphoma. It is likely that you experience other symptoms such as fevers and weight loss if this is the underlying cause. See separate leaflet called Hodgkin's Lymphoma.
Overactive thyroid gland (hyperthyroidism)
Sweating or flushing can be seen with hyperthyroidism. In this condition your thyroid gland is overactive. Other symptoms may include weight loss, feeling warm and hyperactivity. See separate leaflet called Hyperthyroidism (Overactive Thyroid).
There are many other conditions that may cause night sweats. For example, drinking too much alcohol or taking illegal drugs can cause night sweats. See separate leaflets called Alcoholism and Problem Drinking and Recreational Drugs.
A condition called hyperhidrosis causes too much sweating. People with this condition sweat too much in the day and also the night time. See separate leaflet called Excessive Sweating (Hyperhidrosis).
When should you see a doctor about your night sweats?
You should see your doctor if you have regular night sweats. In addition, if you also have a high temperature (fever) or other symptoms, such as unexplained weight loss, you should also go to your doctor. Your doctor may arrange for you to have some tests such as blood tests or even X-rays to determine the cause of your night sweats. The treatment of your night sweats will be dependent on the underlying cause.
Further reading and references
; NICE Guideline (updated August 2017)
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Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.