How to Check Your Energy Level in the Body

There is no single meter on your body that reads “energy level: 73%,” but there are several reliable ways to get a read on how well your body is producing and using energy, ranging from simple questionnaires you can fill out at home to blood panels and clinical tests. The approach that makes the most sense for you depends on whether you’re trying to track day-to-day fluctuations, figure out why you feel chronically drained, or optimize athletic performance. Each method captures a different slice of a complicated picture, and combining a few of them gives you the clearest view.

Start With What You Actually Feel

The most accessible way to check your energy level is to formally assess how you feel, and that sounds obvious, but there’s a reason clinicians use structured questionnaires rather than just asking “are you tired?” Fatigue is a slippery concept. Researchers have found that people frequently confuse fatigue with sleepiness, and the two have different causes and different solutions. A systematic review of studies using the Epworth Sleepiness Scale in multiple sclerosis patients found that sleepiness (difficulty staying awake) was less prevalent and less severe than fatigue (a deeper sense of exhaustion), yet the two terms are routinely used interchangeably in both clinical practice and everyday conversation.¹ This matters because if you misidentify your problem, you chase the wrong fix.

Validated fatigue scales exist precisely to cut through that confusion. The Fatigue Assessment Scale (FAS), for example, is a ten-item questionnaire that has shown strong internal consistency and test-retest reliability across different patient populations.² Other tools like the Fatigue Severity Scale and the Multidimensional Fatigue Inventory break down your tiredness into physical, mental, and motivational components. You can find many of these scales freely available online. Filling one out on a regular basis gives you a trackable number instead of a vague sense that “I’ve been more tired lately.” Even if you never share the results with a doctor, having a consistent baseline makes it much easier to notice when something changes.

One thing these scales highlight is that “low energy” is not a single problem. It can mean physical fatigue (your muscles feel heavy), cognitive fatigue (you can’t concentrate), motivational fatigue (you can’t make yourself start tasks), or excessive sleepiness (you’re nodding off). Each of these points toward different underlying causes and different next steps in checking your energy.

Blood Tests That Flag Common Energy Thieves

If your fatigue is persistent rather than situational, a basic blood panel can catch several of the most common medical reasons for low energy. Your doctor will typically check a few key markers.

Iron and ferritin: Iron disorders are one of the most prominent examples of conditions where fatigue is a leading symptom. Whether iron levels are too low or too high, tiredness tends to be one of the first things people notice.³ A ferritin level (which reflects iron stores) is often more informative than just looking at hemoglobin, because your stores can be depleted well before you become formally anemic.
Thyroid hormones (TSH and free T4): An underactive thyroid is a classic and very treatable cause of fatigue. Research has found a direct relationship between elevated TSH and fatigue severity, with fatigue scores improving after six months of thyroid hormone replacement therapy.⁴
Blood glucose and HbA1c: Unstable blood sugar can produce energy crashes. That said, the relationship between long-term blood sugar control and fatigue perception is not as straightforward as many people assume. One study of people with type 2 diabetes found that baseline HbA1c (a marker of average blood sugar over several months) was not related to self-reported general fatigue, suggesting that real-time glucose swings may matter more than average glucose levels.⁵
B vitamins, vitamin C, magnesium, and zinc: These micronutrients all play recognized roles in energy production at the cellular level. Deficiencies in any of them can manifest as fatigue, reduced cognitive function, or both.⁶ Vitamin B12 deficiency in particular is common in older adults and vegetarians and is easy to miss without a blood test.

A standard blood panel won’t tell you everything, but it’s the single highest-yield step if you suspect something medical is going on. Many of these conditions are common, inexpensive to test for, and highly treatable.

Measuring How Your Body Burns Fuel

If you want a direct measurement of your body’s energy expenditure rather than a blood snapshot, the gold-standard clinical tool is indirect calorimetry. This involves breathing into a device that measures how much oxygen you consume and how much carbon dioxide you produce. From those two numbers, a clinician can calculate how many calories your body is burning at rest (your basal metabolic rate) and what mixture of fats and carbohydrates you’re using for fuel.⁷

This is particularly useful if you suspect your metabolism is unusually slow or fast, or if you’re trying to fine-tune nutrition for weight management. Indirect calorimetry is considered the most sensitive and accurate noninvasive way to measure energy expenditure in an individual.⁸ It’s commonly available in hospital nutrition departments and some sports-medicine clinics. The test itself is straightforward: you lie still for about 15 to 30 minutes while breathing through a mask or canopy. The downside is that it’s not widely offered in primary care offices, and the predictive equations doctors sometimes use as shortcuts (plugging your height, weight, age, and sex into a formula) can be inaccurate, especially in people with overweight or obesity.⁹

For most people, the practical takeaway is that if generic calorie calculators seem to give you numbers that don’t match your experience (you gain weight eating what should be a deficit, or you lose weight unexpectedly), an indirect calorimetry test can give you a real number instead of an estimate.

Fitness Testing and Aerobic Capacity

Your body’s ability to produce energy during physical activity comes down largely to your aerobic capacity, typically measured as VO2max, the maximum rate at which your body can take in and use oxygen. This is probably the single best objective indicator of your overall physical energy potential. A higher VO2max means your muscles can pull more oxygen from your bloodstream and use it to generate energy, which translates into feeling less winded and less fatigued during everyday tasks.

Research has shown that VO2max is closely linked to the ability to resist fatigue during repeated bouts of intense activity. In a study of female recreational athletes, those with higher aerobic capacity showed a greater aerobic contribution during repeated sprints and a significantly better ability to maintain power output.¹⁰ This isn’t just relevant for athletes. Your aerobic fitness sets a ceiling on how much energy your body can mobilize, and people with low aerobic capacity tend to feel drained by activities that healthier individuals barely notice.

The most precise VO2max measurement comes from a graded exercise test on a treadmill or stationary bike, typically done in a sports-medicine lab with a metabolic cart (similar equipment to indirect calorimetry, but done while exercising to exhaustion). A six-week endurance training program can produce measurable improvements in both VO2max and the aerobic capacity of individual muscles.¹¹ Many fitness watches now estimate VO2max from heart rate data during outdoor runs, and while those estimates aren’t lab-grade, they’re useful for tracking trends over time. If your estimated VO2max is climbing, your body’s energy-production capacity is improving.

What Wearables Can and Cannot Tell You

Consumer wearables have become the most popular way people try to monitor their energy, and some of the data they provide is genuinely informative. Heart rate variability, or HRV, measures the tiny fluctuations in timing between heartbeats and reflects how well your autonomic nervous system is balancing rest-and-recover mode against fight-or-flight mode. HRV has emerged as an important indicator for both physical and mental status.¹² A consistently low HRV, especially one that drops from your personal baseline, can signal overtraining, poor recovery, high stress, or illness. Many smartwatches now calculate a daily “readiness” or “body battery” score that leans heavily on HRV, sleep quality, and recent activity.

Continuous glucose monitors (CGMs), once reserved for diabetes management, are now marketed to the general population as energy-tracking tools. The logic is straightforward: if your blood sugar crashes after a meal, you’ll feel tired, and a CGM can show you those crashes in real time. Glucose levels are influenced by nutrient composition, meal timing, physical activity, circadian rhythm, and cortisol, so the data from a CGM can help you identify which meals or habits lead to energy dips.¹³

The frontier of wearable energy monitoring involves combining multiple data streams. Research on fatigue monitoring has demonstrated that wearable devices can identify fatigue with good accuracy when signals from multiple sensors are combined using machine learning models.¹⁴ In practice, though, most consumer devices are still giving you a rough approximation. HRV can tell you something meaningful about recovery, and a CGM can reveal post-meal glucose patterns, but neither one is literally reading your “energy level.” They’re useful proxies, not direct measurements.

Cortisol and the Stress-Energy Connection

Cortisol is the hormone most tightly linked to the daily rhythm of alertness and energy, and measuring it can reveal problems that blood tests for iron or thyroid might miss. The cortisol awakening response (CAR), a sharp spike in cortisol that occurs in the first 30 to 45 minutes after waking, is a distinct feature of the body’s circadian rhythm and appears to be connected to how well you cope with the day’s demands. Research in healthy adults has found that a stronger morning cortisol rise is associated with better coping with same-day stress.¹⁵

For athletes and people with physically demanding routines, cortisol patterns can also serve as an early warning of overtraining. A study of athletes undergoing seven days of intensive training found that changes in the awakening cortisol response and midnight cortisol were sensitive markers of training-induced fatigue. Athletes who experienced the largest performance drops after the training block also showed the most disrupted cortisol rhythms.¹⁶ Salivary cortisol kits are available for home use, and some functional medicine practitioners order four-point cortisol tests (samples taken at waking, midday, evening, and bedtime) to map the shape of your daily cortisol curve. A flattened curve, one that doesn’t spike properly in the morning or stays elevated at night, can help explain persistent tiredness even when other lab work looks normal.

Your Internal Clock Sets the Baseline

Before you decide your energy is abnormally low, it’s worth considering whether you’re simply measuring it at the wrong time of day for your body. People have genuinely different chronotypes, and these aren’t just preferences. Morning types and evening types show measurably different patterns of energetic arousal across the day. In a study comparing extreme morning types and evening types, the morning group showed higher energy levels from about 8:00 a.m. to 5:00 p.m., while the two groups showed no difference in energy during later hours.¹⁷ If you’re a natural evening type and you assess your energy at 9:00 a.m. every day, you’ll consistently conclude it’s low, but the problem isn’t your energy system. It’s your schedule.

The Morningness-Eveningness Questionnaire is one of the most accepted tools for determining your chronotype.¹⁸ Knowing your chronotype helps you interpret your energy patterns more accurately and organize your most demanding tasks around your natural peak. It also helps you avoid the common trap of comparing your energy to someone else’s when the two of you are running on different internal clocks.

Hydration Is Easier to Miss Than You Think

Dehydration is one of the simplest and most overlooked causes of low energy. You don’t have to be visibly parched or exercising in the heat for it to affect you. A study that progressively restricted fluid intake in healthy women found that even mild dehydration produced decreased alertness and increased fatigue, sleepiness, and confusion.¹⁹ The effects on sleepiness and fatigue were the most consistent findings.

Checking hydration is low-tech. Urine color is a reasonable proxy: pale straw means adequate, dark yellow means you’re behind. Body weight before and after exercise gives you sweat loss. And simply tracking how much you drink over a day or two can reveal that you’re routinely under-hydrating. If your energy consistently dips in the afternoon and you haven’t had water since your morning coffee, hydration is the cheapest and fastest variable to rule out.

Your Gut’s Role in the Energy Equation

An area of growing research is the connection between your gut microbiome and how much energy you actually extract from food. Not all of the calories in your meals end up fueling your cells. Your gut bacteria consume some of that energy themselves, and the balance depends on the composition of your microbial community and what you eat. A randomized clinical trial comparing a microbiome-enhancing diet (high in fiber, fermented foods, and plant diversity) with a typical Western diet found that the microbiome-friendly diet shifted more energy toward the microbes. It increased production of short-chain fatty acids in both the stool and the bloodstream and also increased microbial biomass.²⁰

This doesn’t mean a high-fiber diet drains your energy. Short-chain fatty acids like butyrate actually fuel the cells lining your gut and have anti-inflammatory effects that may reduce fatigue over time. But it does mean the relationship between what you eat and how energized you feel is mediated in part by the trillions of microbes in your intestines, and two people eating the same meal can extract different amounts of usable energy from it. Microbiome testing is commercially available now, though the science of translating a microbiome readout into specific dietary advice is still in its early stages.

When Low Energy Signals Something Deeper

For most people, low energy reflects some combination of poor sleep, nutrient gaps, deconditioning, stress, or dehydration. But for a subset of people, fatigue is the central symptom of a condition that standard blood tests won’t catch. Myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) involves a distinctive pattern called post-exertional malaise, where physical or mental exertion triggers a disproportionate crash in energy that can last days.

One of the more interesting diagnostic developments for ME/CFS is the two-day cardiopulmonary exercise test (2-day CPET). The idea is simple: you do a standard exercise test on day one to measure your baseline, which also provokes post-exertional malaise. Then you repeat the same test 24 hours later and compare the results.²¹ In healthy people, performance on the second test is roughly the same. In people with ME/CFS, performance drops measurably. Research has identified a decrease in work rate at the ventilatory threshold of roughly 6 to 10 percent on the second day, which may represent an objective biomarker for the condition.²² This is significant because ME/CFS has long been dismissed as subjective or psychological, and the two-day CPET provides objective, physiological evidence of abnormal energy production after exertion.²³

The 2-day CPET is not widely available and is mostly used in research settings and specialized clinics. But it’s worth knowing about if your fatigue is severe, worsens with activity, and hasn’t been explained by standard testing. It’s also a reminder that “energy level” isn’t just about willpower or sleep hygiene. Your body has real, measurable energy-production systems, and when those systems malfunction, the evidence shows up on the right tests.

Why Brain Fatigue and Body Fatigue Feel Different

Many people notice that their energy can be depleted mentally without any physical exertion, or vice versa. This isn’t just perception. Central fatigue, the kind originating in the brain, involves neurotransmitter systems that are distinct from the metabolic pathways driving your muscles. Research has shown that brain chemicals including serotonin, dopamine, and noradrenaline all play roles in the onset of fatigue, but no single neurotransmitter system appears to be solely responsible.²⁴ Dopamine-related drugs seem to help most when fatigue is caused by overheating, while serotonin-related drugs haven’t shown consistent performance effects. The picture is messy and still incomplete.

The practical implication is that if your energy problem is primarily cognitive, looking at sleep quality, stress, hydration, and blood glucose may be more productive than checking your VO2max or ferritin. And if your fatigue is purely physical, a micronutrient panel and fitness assessment will likely be more revealing than a cognitive battery. Identifying which type of fatigue you’re dealing with is itself a form of checking your energy level, and it points you toward the right next test rather than shotgunning every lab panel available.