Circadian (daily) rhythms
Throughout the day, the body's temperature indicators are subject to strictly defined dynamics. The normal amplitude of such daily changes is in the range of 0.5–0.7 °C. This process is governed by complex physiological regulatory mechanisms.
The main drivers of circadian rhythms are:
- Changes in the tone of the autonomic nervous system (ANS).
- Daily dynamics of the secretion of various hormones regulating anabolic and catabolic processes.
The regularity of changes is clearly visible when analyzing rectal temperature graphs:
- Waking period: Begins in the morning hours (06:00–08:00) and continues until late evening (23:00–24:00). During this interval, a temperature plateau forms — values stably remain at a high level, in the range of 36.8–37.0 °C. The maximum peak is usually recorded in the second half of the day.
- Nighttime sleep period: Upon falling asleep (around midnight), there is a sharp drop in values. The coldest point, or nadir, is reached in the middle of the night and towards morning (04:00–06:00), when values drop to a minimum of 36.2–36.4 °C. Immediately after waking up, a rapid rise to daytime values is observed.
Cyclical changes in women
The female body is characterized by pronounced monthly fluctuations, which are conveniently tracked by measuring rectal temperature. These changes are inextricably linked with the phases of the menstrual cycle.
- Menstruation: Characterized by a decrease in temperature indicators.
- Follicular (first) phase: Temperature consistently remains at a low baseline level.
- Mid-cycle (ovulation): Egg release occurs, which serves as a trigger for the subsequent thermal shift.
- Second phase: After ovulation, body temperature naturally increases by 0.5–1.0 °C, which is associated with active hormonal restructuring preparing the body for a potential pregnancy.
Other physiological and pathological factors
In addition to daily and monthly rhythms, the body's thermal balance is influenced by numerous situational factors associated with lifestyle and physiological state:
- Food intake: After eating, heat production occurs due to the specific dynamic action of food. This is because the gastrointestinal tract expends significant energy on digesting and assimilating nutrients.
- Physical activity: Intense muscular work is always accompanied by increased heat production and, as a result, a rise in body temperature.
- Psychoemotional state: Nervous excitement and acute stress can lead to a temporary increase in temperature indicators.
- Special states: Pronounced thermoregulation changes are characteristic of pregnancy and childbirth.
- Pathology: It is important to distinguish physiological fluctuations from infectious diseases, in which elevated temperature (fever) is a defensive reaction.
Briefly: answers to topical questions
Where is the thermoregulation center located that controls the circadian rhythms of body temperature?
The thermoregulation centers that maintain constant body temperature are located in the hypothalamic region, i.e., in the hypothalamus.
By sources:
- the anterior hypothalamus is primarily responsible for heat loss;
- the posterior hypothalamus is primarily responsible for heat production;
- the suprachiasmatic nucleus is the main circadian pacemaker ("biological clock") of the body.
Daily temperature fluctuations belong to circadian rhythms; their physiological mechanisms include the dynamics of anabolic and catabolic hormone secretion and changes in autonomic nervous system tone.
Which hormone causes an increase in rectal temperature during the second phase of the menstrual cycle?
The increase in rectal (basal) temperature during the second phase of the menstrual cycle is due to the thermogenic action of progesterone.
This hormone is secreted by the corpus luteum formed in the ovary during the luteal phase. Progesterone acts on the thermoregulatory center in the hypothalamus, increasing basal metabolism and basal body temperature (usually by 0.4–0.6 °C higher than the values of the first phase). The temperature remains elevated until the end of the cycle and drops before the onset of the next menstruation when hormone levels fall.
What is the pathogenetic difference between fever and a physiological temperature rise?
The pathogenetic difference is that in fever, the "set point" of the thermoregulation center is shifted to a higher level under the action of endogenous pyrogens on the neurons of the preoptic area of the hypothalamus.
At the same time, the normal core body temperature is perceived as low, mechanisms for increasing heat production and reducing heat loss are activated; subjectively, a person feels chills.
Physiological increases and fluctuations in temperature are described in sources as changes associated with ordinary factors: circadian rhythms, menstrual cycle, food intake, physical activity, nervous excitement, pregnancy, and childbirth. For daily fluctuations, the dynamics of anabolic and catabolic hormone secretion and changes in ANS tone are indicated; the rise after food intake is due to the specific dynamic action of food; in the second phase of the menstrual cycle, temperature rises under the influence of progesterone.
Temperature rise factors — FMS: Food (digestion), Muscles (work), Stress (emotions).
Class and exam questions
At what time of day is human body temperature at its minimum?
The minimum (nadir) is reached during deep night sleep, usually between 04:00 and 06:00 AM.
Why might you feel hot after a hearty lunch?
This is due to the specific dynamic action of food — the body spends additional energy on the work of the gastrointestinal tract, which is accompanied by heat release.
By how many degrees can temperature change during the day?
Normally, the physiological amplitude of daily circadian fluctuations ranges from 0.5 to 0.7 °C.
What else to study on this topic
- Mechanisms of the specific dynamic action of food
- The role of the autonomic nervous system in regulating heat exchange
- Hormonal control of thermogenesis in the second phase of the cycle
- Catabolic and anabolic processes as a source of heat
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