Fever increases, pain and discomfort intensify, and coughing or asthmatic attacks are more frequent and intense than during the day. The feeling that at night the symptoms of illness worsen is not a subjective perception but is based on medical evidence. The influence of circadian rhythm has been known in the medical community for over 150 years, but over time, a number of studies have investigated in depth the link between circadian rhythm and the internal processes of the human body, as well as how it amplifies the symptoms of certain diseases.
Why does temperature (fever) rise at night
Normal temperature and fever in children and adults
Circadian rhythm
The circadian rhythm encompasses the totality of biochemical, physiological processes that occur in the body during a 24-hour cycle, in an alternating form: sleep/wake, night/day.
Internally, there is a double synchronization, on the one hand between the biochemical-physiological processes specific to the human body, and on the other hand, between these processes and factors that affect the external environment, such as light. Exposure to light influences the amount of melatonin, a hormone secreted by the pineal gland (epiphysis) that induces sleep. The body produces a certain amount of melatonin directly proportional to the day/night, light/dark dynamic. Thus, there are higher values of this hormone after dark, during the night, and lower values in the early hours of the morning.
As part of the circadian rhythm, body temperature also rises or falls depending on the time of day, reaching its lowest values in the morning and its highest values at night.
Body temperature is the result of two processes, thermogenesis and thermolysis (the production and release of heat by the body) and does NOT remain constant throughout the day, values can vary by half a degree plus +, minus -, even in clinically healthy people.
Why does temperature (fever) rise at night
The night is the time when the body replenishes its resources. Certain internal functions and processes go into a resting state that allows the body to restore its working capacity (physical and mental) for the next day.
The hypothalamus, an almond-sized endocrine gland located at the base of the brain, controls body temperature. On the one hand, it responds to stimuli transmitted through the blood (such as blood temperature), and on the other hand, it receives information from other tissues (skin) and uses all this information, as well as other chemical signals transmitted by the body, to regulate temperature.
It is also the hypothalamus that controls the circadian rhythm via the suprachiasmatic nucleus (SCN). The SCN ‘picks up’ light/dark signals with the help of nerve cells in the eyes. These internal processes are responsible for fluctuations in body temperature, higher at night, lower in the early morning and early part of the day, and they do not stop during illness.
The symptoms of respiratory illnesses, including fever, as a response of the immune system to the action of pathogens, manifest themselves more acutely during the night, influenced also by these absolutely normal day/night variations.
At the same time, the circadian rhythm influences the activity of the immune system, where fever is the body’s defense shield against pathogens. Pyrogens, the substances that trigger fever, are released into the bloodstream by leukocytes (white blood cells) when they detect any type of ‘intruder’ pathogen such as viruses, bacteria, fungi, and parasites.
In addition to leukocytes, pyrogens can also come directly from the pathogen itself or from infected tissue. Regardless of the source of the pyrogens, the result is the same, they cause the hypothalamus to raise body temperature.
Normal temperature and fever in children and adults
Normal temperature values for newborns and young children: 36.1 °C – 37.8 °C
Adults: 36°C – 37°C
Elderly: 35°C – 36°C
Fever is considered to be a temperature exceeding 38°C
Moderate fever is between 38°C – 39°C
High fever 39°C – 40°C
Hyperpyrexia above 40°C – is a very high fever and is a medical emergency.
Learn about Fever in children – from A to Z
Why pain is worse at night
Somatic pain is generated by the reaction of neuronal receptors to noxious stimuli. Various studies over time have shown a relationship between circadian rhythm and pain response, with the algic threshold (pain experienced) varying throughout the day, with the lowest thresholds occurring in the early morning and the highest thresholds in the late evening.
For example, toothache is the most common reason for early morning visits to dental surgeries. The answer lies in the fact that the threshold for tooth pain on electrical stimulation is lower in the latter part of the night and higher in the early morning.
Among patients with chronic musculoskeletal conditions, a time lag was also observed between the times of a day when the pain was more acute. In the case of osteoarthritis, the most violent degree of pain was in the evening, and in patients with rheumatic diseases, pain and muscle stiffness were felt more strongly at night and in the early morning.
The specific symptoms of a cold are distressing during the day (sore throat, stuffy nose, coughing), but at night they can turn into a real nightmare, being aggravated by both the position of the body and the activity of the immune system. The activities of the day do not allow attention to be concentrated in one area, that of the clinical manifestations caused by the virus, but, at the end of the day, the body relaxes and the active mechanisms of the immune system are put into operation. On the other hand, congestion of the nose is favored by the lying position, hence the difficulty in breathing, and the impossibility of falling asleep or maintaining sleep.
References:
- Fever Incidence Is Much Lower in the Morning than the Evening: Boston and US National Triage Data, Charles Harding, Francesco Pompei, Samantha F. Bordonaro, Daniel C. McGillicuddy, Dmitriy Burmistrov, Leon D. Sanchez – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7390559/
- Pain at night, F. Dudley Hart, R.T Taylor, E.C Huskisson – https://www.sciencedirect.com/science/article/abs/pii/S0140673670917046
