A basal body temperature that seems unusually low is, in most cases, not unusual at all. The old textbook figure of 98.6 °F (37.0 °C) was established in the 1860s and no longer reflects what researchers find when they measure large groups of people today. Average human body temperature has been steadily declining for over a century, and a waking temperature in the mid-97s or even high 96s is well within the range seen in healthy adults. That said, a genuinely low reading can sometimes point to thyroid problems, nutritional gaps, medication effects, or simply an unreliable thermometer.
The 98.6 Benchmark Is Outdated
The number most people carry around in their heads comes from a German physician named Carl Wunderlich, who published it in 1868 based on armpit measurements from thousands of patients. Since then, body temperature across entire populations has drifted downward. A large analysis using data spanning nearly two centuries of American health records found a steady decline in temperature by birth cohort: roughly 0.03 °C per decade for both men and women, adding up to about 0.6 °C lower in men born in the late twentieth century compared with those born in the early 1800s.1PubMed Central. Decreasing human body temperature in the United States since the Industrial Revolution That shift means the “normal” most of us were taught is probably a few tenths of a degree too high for a modern adult.
Why the decline? The leading theories involve reduced chronic inflammation (fewer untreated infections, better sanitation, wider antibiotic use) and changes in metabolic rate driven by more climate-controlled environments and less physical labor. None of this means your body is broken. It means the reference point itself has moved, and a resting oral temperature around 97.5 °F is perfectly compatible with good health.
How You Measure Makes a Real Difference
Before investigating medical explanations, it is worth questioning the number on your thermometer. Oral thermometers are the most common tool people use at home, yet they are notoriously sensitive to outside influences. A systematic review found that oral temperature cannot reliably reflect true core body temperature because it is affected by ambient air temperature, where the probe sits under your tongue, and whether you recently drank something hot or cold.2PubMed Central. Is oral temperature an accurate measurement of deep body temperature? A systematic review Breathing through your mouth on a cold morning, for instance, can drag an oral reading down by a noticeable margin.
Ear (tympanic) thermometers carry their own quirks. They detect infrared radiation from the ear canal and then convert that signal through an algorithm to estimate core temperature, so the reading is an estimate rather than a direct measurement.3PubMed Central. Oral and Tympanic Membrane Temperatures Are Inaccurate to Identify Fever in Emergency Department Adults A small amount of earwax, an incorrect angle, or a cold outer ear can all skew the result. If you consistently get readings that seem low, try measuring under the same conditions each time: same thermometer, same technique, same time of day, at least 15 minutes after eating or drinking.
Time of Day and Your Internal Clock
Your body temperature is not a fixed number. It follows a circadian rhythm, dipping to its lowest point in the early morning hours (typically between 4 and 6 a.m.) and peaking in the late afternoon or early evening.4PubMed Central. Circadian rhythmicity of body temperature and metabolism The difference between the daily low and high can be around 1 °F or more. If you are taking your basal temperature right when you wake up, you are catching it near its lowest natural point. That is the whole purpose of basal body temperature tracking, but it also means a reading of 97.2 °F at 6 a.m. might climb to 98.2 °F by mid-afternoon in the very same person.
Brown adipose tissue, a type of fat that generates heat, appears to follow its own daily rhythm. Research has shown that cold-stimulated calorie burning and fat oxidation are higher in the morning than in the evening, and that the gap is especially pronounced in people with more active brown fat.5international journal of obesity. Diurnal variations of brown fat thermogenesis and fat oxidation in humans So your body’s heat engine itself runs on a schedule, and that schedule contributes to why early-morning temperatures tend to look low.
Thyroid Function Is the Classic Culprit
When people worry about a persistently low temperature, the thyroid gland is usually the first thing a doctor investigates, and for good reason. Thyroid hormones are central to thermogenesis, the process by which your body generates heat. Animal research has demonstrated this starkly: mice lacking one of the thyroid hormone receptors run a body temperature about 0.5 °C below normal even though they otherwise appear healthy.6PubMed Central. Abnormal heart rate and body temperature in mice lacking thyroid hormone receptor alpha 1
In humans, an underactive thyroid (hypothyroidism) slows metabolism across the board, and feeling cold or registering a lower-than-expected temperature is one of the hallmark complaints. The condition is common and treatable. If a low basal temperature comes with fatigue, unexplained weight gain, dry skin, or hair thinning, a simple blood test for TSH and free T4 can confirm or rule out the diagnosis. Subclinical hypothyroidism, where hormone levels are only mildly off, may also drag temperature down without producing dramatic symptoms.
Where You Are in Your Menstrual Cycle
For anyone who menstruates, the phase of the cycle has a predictable effect on basal body temperature. During the first half of the cycle (the follicular phase), when estrogen is dominant and progesterone is low, temperatures run lower. After ovulation, progesterone surges and pushes core body temperature up by roughly 0.3 to 0.7 °C compared with the pre-ovulatory phase.7PubMed Central. Temperature regulation in women: Effects of the menstrual cycle This is the thermal shift that fertility-awareness methods rely on to estimate ovulation timing.
If you are consistently checking your temperature during the follicular phase and comparing it to a generic “normal,” the mismatch is expected. One study specifically confirmed that basal body temperature values are lower during cycle phases when sex hormone levels are low.8PubMed. Changes in basal body temperature and simple reaction times during the menstrual cycle And interestingly, the brown adipose tissue activity that contributes to heat generation does not appear to change much between the follicular and luteal phases, suggesting that the temperature shift is driven more by hormonal resetting of the body’s thermostat than by changes in heat production capacity.9PubMed Central. Association of brown adipose tissue activity with circulating sex hormones and fibroblast growth factor 21 in the follicular and luteal phases in young women
Hormonal contraceptives add another layer. Women using oral contraceptives tend to have a slightly higher baseline core temperature, with one study recording resting temperatures of about 37.5 °C in oral-contraceptive users compared with about 37.0–37.2 °C in naturally cycling women.10PubMed. Response of women using oral contraception to exercise in the heat If you recently stopped hormonal birth control, the drop in baseline temperature you notice may simply be your body returning to its unmedicated pattern.
Eating Too Little Lowers Your Temperature
Calorie restriction has a direct and well-documented cooling effect. Your body treats a caloric deficit as a signal to conserve energy, and one of the fastest ways to conserve is to dial down heat production. A study of lean, weight-stable adults who had practiced long-term calorie restriction found that their 24-hour core body temperature, both during the day and at night, was significantly lower than that of people eating normally or exercising heavily without restricting calories.11PubMed Central. Long-term calorie restriction, but not endurance exercise, lowers core body temperature in humans Exercise alone did not produce the same drop, which means it is not just about being thin; it is specifically about energy availability.
A randomized trial confirmed the pattern on a shorter timescale: after six months of calorie restriction, participants had measurably reduced 24-hour core body temperature along with lower sleeping energy expenditure.12JAMA. Effect of 6-Month Calorie Restriction on Biomarkers of Longevity, Metabolic Adaptation, and Oxidative Stress in Overweight Individuals: A Randomized Controlled Trial And the cooling does not snap back immediately once you eat more. Animal research has shown that core temperature remains suppressed even during the refeeding and weight-regain phase, suggesting the body clings to its energy-saving mode for a while after restriction ends.13PubMed. Low 24-hour core body temperature as a thrifty metabolic trait driving catch-up fat during weight regain after caloric restriction
For anyone on a strict diet, intermittent fasting protocol, or recovering from a period of undereating, a low basal temperature is a tangible sign that your metabolism has downshifted. It is not dangerous on its own, but it is your body telling you it is in conservation mode.
Age and Muscle Mass
Body temperature tends to drift downward with age. A systematic review and meta-analysis found that older adults run about 0.17 °C cooler than younger adults on average, with the effect being more pronounced in men and in studies using oral measurements.14PubMed. The Effect of Aging on Body Temperature: A Systematic Review and Meta-Analysis A large cross-sectional study of over 18,000 individuals put the gap at about 0.3 °F between the oldest and youngest groups after adjusting for body mass and other factors.15The Journals of Gerontology: Series A. Is Older Colder or Colder Older? The Association of Age With Body Temperature in 18,630 Individuals
Part of this is tied to changes in body composition. Skeletal muscle is metabolically active tissue that generates heat even at rest. As people age and tend to lose muscle mass, overall metabolic heat production declines.16PubMed Central. Effect of Exercise Training on Body Temperature in the Elderly: A Retrospective Cohort Study This means that a lower temperature in a 70-year-old is not inherently worrisome. But it also means the margin between “their normal” and clinically significant hypothermia is narrower, so older adults need to pay more attention to staying warm in cold environments.
Iron Deficiency and Its Thyroid Connection
Iron deficiency is one of the most common nutritional shortfalls worldwide, and it has a surprisingly direct link to body temperature. The connection goes beyond just having less oxygen-carrying capacity in the blood. Research has shown that iron deficiency impairs the conversion of the thyroid prohormone T4 into its active form T3, the hormone most directly involved in heat generation. In iron-deficient animals, T3 levels in the cold were significantly lower than in controls, and correcting the iron deficiency with supplementation normalized the T3 response within about six days.17PubMed. Hypothermia in iron deficiency due to altered triiodothyronine metabolism
A follow-up study confirmed that cold sensitivity in iron-deficient animals tracked closely with their impaired thyroid responses, finding a significant positive correlation between iron status and the rise in thyroid hormones during cold exposure.18PubMed. Effect of iron-deficiency anemia on hormone levels and thermoregulation during cold exposure The practical takeaway: if your temperature is low and you also experience fatigue, pallor, or shortness of breath, getting your iron levels checked alongside your thyroid panel makes sense. Fixing a straightforward iron deficiency can restore your thermal regulation without needing thyroid medication.
Medications That Alter Your Thermostat
Several classes of prescription drugs can nudge body temperature in one direction or another, often as an overlooked side effect. A systematic review examining how common medications affect thermoregulation found that drugs with strong anticholinergic properties (a category that includes certain antihistamines, bladder medications, and older antidepressants) substantially reduced sweating and altered core temperature regulation.19eClinicalMedicine. Effects of prescription medications on thermoregulation and core temperature during heat stress: a systematic review and meta-analysis Beta-blockers, particularly non-selective types, also shifted core temperature modestly. Diuretics can deplete blood volume in a way that affects how efficiently your circulatory system distributes heat.
Most of this research focuses on the risk of overheating during heat stress, but the underlying point applies to baseline temperatures as well: if a drug is altering the way your body produces, distributes, or dissipates heat, your resting temperature may read differently than it did before you started the medication. If your low temperatures coincided with starting a new prescription, the drug is worth discussing with your doctor as a possible contributor.
Blood Sugar, Brain Wiring, and Other Medical Causes
Prolonged low blood sugar can cause body temperature to fall, though this is more relevant to people with diabetes using insulin or other glucose-lowering drugs than to the general population. A clinical case report documented a patient whose temperature dropped steadily during a bout of severe hypoglycemia and only recovered once blood sugar was corrected with a dextrose infusion.20PubMed Central. Hypothermia as a forgotten sign of prolonged severe hypoglycaemia The mechanism makes sense: producing heat costs energy, and when glucose runs critically low, the body curtails nonessential spending.
At the neurological level, temperature regulation is governed by a small cluster of neurons in the hypothalamus. Optogenetic experiments in animals have shown that activating one set of these neurons drops body temperature, while inhibiting them causes a spike into fever-range temperatures.21PubMed Central. A hypothalamic circuit that controls body temperature Anything that affects hypothalamic function, from traumatic brain injury to certain tumors to pituitary disorders, can theoretically alter your temperature set point. These are rare causes, but they explain why some people with otherwise unexplained low temperatures eventually get diagnosed with a central nervous system issue.
Chronic fatigue syndrome and other conditions that involve dysregulation of the autonomic nervous system are sometimes associated with low basal temperatures as well, though the research in those areas is less definitive and the causal direction is not always clear.
When to Actually Worry
A low-but-stable basal body temperature that you have always had, or that matches your age, diet, and cycle phase, is rarely a medical emergency. The situations where a low temperature warrants prompt attention include:
- Acute drop: Your temperature falls noticeably from your usual baseline over a short period, especially if accompanied by confusion, sluggishness, or shivering that stops.
- Symptoms alongside it: Persistent fatigue, unexplained weight changes, hair loss, constipation, or feeling cold when others are comfortable point toward thyroid or metabolic issues worth testing.
- Readings below 95 °F (35 °C): This enters the clinical definition of hypothermia, which requires medical evaluation regardless of circumstances.
- Post-surgical or post-illness context: A low temperature after a major infection or procedure can indicate the body’s thermoregulatory system is still recovering, or in some cases signal a secondary infection where the immune response is muted.
For most people who land on this question because their fertility tracker or morning thermometer consistently shows a number in the 97s or even high 96s, the answer is reassuringly mundane. Average body temperature is lower than the old standard, oral thermometers are imperfect tools, and early morning is the coldest part of your day. Start with those explanations before assuming something is wrong, and if you have other symptoms that concern you, a thyroid panel and a complete blood count are the two simplest screening tests your doctor can order.
Sleep and the Nightly Temperature Dip
The relationship between sleep and body temperature runs deeper than most people realize. Falling asleep is not just something that happens to coincide with cooling down; the drop in core temperature is part of the mechanism that initiates and maintains sleep. Your brain actively promotes heat loss through the skin, especially the hands and feet, as part of the transition into sleep. Research into thermoregulation during sleep describes this whole-organism cooling process as optimized for energy conservation.22PubMed Central. Sleep and thermoregulation Core temperature reaches its daily minimum during the deepest phases of sleep in the early morning hours, which is precisely when most people take a basal body temperature reading.
If you are someone who sleeps in a cold bedroom, sleeps poorly and wakes frequently, or takes your temperature after a particularly deep stretch of sleep, the reading can sit even lower than your personal average. People who work night shifts or have irregular sleep schedules may also see unusual temperature readings because their circadian temperature rhythm gets shifted or flattened. The temperature you record upon waking is always a product of both your underlying physiology and whatever your sleep did to it that particular night.