Your body temperature shifts throughout the day based on a surprisingly long list of factors, from the time you wake up to the medications you take. The old standard of 98.6°F is outdated. Stanford Medicine researchers found that today’s average body temperature is closer to 97.9°F, with a normal range of about 97.3°F to 98.2°F. That number has been dropping roughly 0.05°F per decade since the 19th century, likely because improvements in health and living conditions have reduced chronic inflammation.
Understanding what pushes your temperature up or down helps you interpret what’s normal for your own body and recognize when something is off.
Your Internal Thermostat
A small region in your brain called the hypothalamus acts as your body’s thermostat. It constantly monitors your blood temperature and triggers automatic responses to keep you in a safe range. When your temperature climbs too high, the hypothalamus activates sweat glands and redirects blood toward the skin so heat can escape. It also dials back your metabolic rate, which reduces internal heat production.
When your temperature drops too low, the opposite happens. Blood vessels near the skin constrict to keep warm blood closer to your organs. Your adrenal glands release stress hormones that ramp up metabolism, and your muscles begin contracting involuntarily, which is shivering. Even goosebumps serve a purpose: they trap a thin layer of insulating air against the skin. In infants under six months old, the body also burns a special type of fat called brown fat to generate heat directly, without shivering.
Your Daily Temperature Cycle
Body temperature follows a predictable rhythm tied to your internal clock. It reaches its lowest point in the early morning hours, typically around 4 to 5 a.m., then gradually rises throughout the day. Data from multiple human studies show that the daily peak occurs roughly 7 to 11 hours after waking, which places it in the late afternoon or early evening for most people. The total swing from low to high ranges from about 0.7°C to 1.6°C (roughly 1.3°F to 2.9°F) depending on the individual.
This cycle persists even when you control for meals, activity, and sleep. It’s driven by the same circadian system that regulates your sleep-wake pattern, which means disruptions like shift work or jet lag can temporarily alter your temperature rhythm.
Physical Activity and Metabolic Rate
Exercise is one of the most powerful short-term drivers of body temperature. When you work out, your muscles generate enormous amounts of heat as a byproduct of energy use. Your core temperature rises rapidly at first, then levels off once your body’s cooling mechanisms catch up and heat loss equals heat production. The size of the increase is proportional to how hard you’re working, not the environment you’re exercising in (within a reasonable range of conditions).
Dehydration makes this worse. As you lose fluid through sweat, your blood volume drops and your body becomes less efficient at sweating and sending blood to the skin for cooling. The result is a higher core temperature for the same level of effort. This is why staying hydrated during exercise matters so much for temperature regulation, not just comfort.
Hormones and the Menstrual Cycle
Hormonal shifts directly influence body temperature. The most well-documented example is the menstrual cycle. After ovulation, progesterone levels rise and push basal body temperature up by about 0.3°C (roughly half a degree Fahrenheit). This elevated temperature holds through the luteal phase until menstruation begins. It’s consistent enough that tracking morning temperature is used as a method of natural family planning.
Thyroid hormones also play a major role. They control your metabolic rate, which is essentially how much heat your body produces at rest. An overactive thyroid can leave you feeling constantly warm, while an underactive thyroid often makes people feel cold even in comfortable environments.
Infection and Fever
When bacteria, viruses, or other pathogens enter the body, immune cells release signaling molecules that travel to the hypothalamus. These signals trigger the production of a chemical called prostaglandin E2, which raises the hypothalamus’s temperature set point. Your body then responds as if it were too cold: you shiver, blood vessels constrict, and your temperature climbs. This is fever, and it’s an intentional defense mechanism. Higher temperatures help immune cells work more efficiently and create a less hospitable environment for many pathogens.
The key signaling molecules involved are produced by immune cells called macrophages when they detect invaders. These molecules reach the brain through a specialized area where the blood-brain barrier is more permeable, allowing the immune system to communicate directly with the thermostat.
Age and Vulnerability
Infants and older adults are both more vulnerable to temperature swings, but for different reasons. Newborns and young children have functional heat production and cooling systems, but those systems are easily overwhelmed. A baby’s small body mass and large surface area relative to weight mean they lose heat quickly, and their ability to sustain a defense against cold or hot conditions is limited. Both hypothermia and overheating can develop fast.
In older adults, the resting body temperature tends to be lower than in younger people. The body’s ability to sense temperature changes also declines with age, so the threshold for triggering sweating or shivering shifts. Responses to heat or cold are often delayed or insufficient. One clinically important consequence: older adults may not mount a typical fever during infection. In some cases, a serious infection produces a below-normal temperature instead, which can mask the severity of the illness.
Medications and Substances
A wide range of medications interfere with your body’s ability to regulate temperature. This is especially important during heat waves or intense physical activity. The main categories include:
- Psychiatric medications: Antidepressants (including SSRIs and older tricyclic types), antipsychotics, stimulants used for ADHD, and mood stabilizers can all impair sweating or raise metabolic heat production.
- Heart and blood pressure medications: Diuretics increase fluid loss, while beta blockers and calcium channel blockers can reduce the heart’s ability to increase blood flow to the skin for cooling.
- Antihistamines: Older-generation allergy medications with anticholinergic properties suppress sweating.
- Thyroid replacement: By increasing metabolic rate, these medications raise baseline heat production.
- Alcohol and stimulants: Alcohol dilates blood vessels and impairs the body’s temperature sensing. Stimulants like amphetamines and cocaine raise metabolic rate and core temperature directly.
If you take any of these and spend time in hot conditions, your risk of heat-related illness is meaningfully higher than someone who doesn’t.
Heat, Cold, and Humidity
Your environment sets the stage for how well your cooling and heating systems work. In dry heat, sweating is highly effective because sweat evaporates quickly, pulling heat away from the skin. As humidity rises, the air holds more moisture and evaporation slows dramatically. This is why 90°F with high humidity feels far more dangerous than 100°F in a desert. Your body is still producing sweat, but it pools on the skin instead of evaporating, so the cooling effect drops.
Cold environments force the body to conserve and generate heat. Wind accelerates heat loss from exposed skin, and water conducts heat away from the body roughly 25 times faster than air at the same temperature. This is why falling into cold water is so much more dangerous than standing in cold air.
Where You Measure Matters
The number on your thermometer depends partly on where you take the reading. Rectal temperature is considered the closest to true core temperature. Armpit (axillary) readings run about 1°C (1.8°F) lower than rectal in people older than one month. Oral readings fall somewhere in between, influenced by recent eating or drinking. Forehead and ear thermometers offer convenience but tend to be less precise.
These differences matter when you’re trying to assess a fever. An armpit reading of 99°F might correspond to an actual core temperature closer to 100.8°F, which changes whether the reading qualifies as a true fever.