Fatigue and body aches showing up together usually point to one of a handful of causes: an infection your immune system is fighting, a nutritional deficiency, a hormonal imbalance, a chronic pain condition, or poor sleep. These two symptoms travel as a pair because they share common triggers, particularly inflammation, low oxygen delivery to muscles, and disrupted energy production at the cellular level. The specific cause depends on how long your symptoms have lasted, what else you’re experiencing, and whether the onset was sudden or gradual.
Infections: The Most Common Short-Term Cause
When fatigue and body aches come on suddenly, an infection is the likeliest explanation. Viruses like influenza, COVID-19, and mononucleosis trigger a system-wide inflammatory response that makes muscles ache and drains your energy. Your immune system redirects resources toward fighting the pathogen, which is why you feel wiped out even if you’re resting in bed. The aches aren’t from muscle damage; they’re a side effect of the signaling molecules your body releases to coordinate the immune response.
Most viral infections resolve within one to two weeks, but mono is a notable exception. Caused by the Epstein-Barr virus, mono leaves most people feeling better within two to four weeks, though fatigue can linger for several additional weeks. In some cases, mono symptoms persist for six months or longer. Bacterial infections like strep throat or urinary tract infections can also cause fatigue and generalized aching, though they tend to come with more localized symptoms as well.
If your symptoms started within the past few days and you also have a fever, sore throat, or congestion, an acute infection is the most probable cause.
Iron Deficiency and Low Vitamin D
Nutritional deficiencies are one of the most overlooked reasons people feel tired and achy for weeks or months without an obvious explanation. Iron deficiency is especially common, and it doesn’t have to progress to full-blown anemia before it causes symptoms. Ferritin levels below 30 micrograms per liter indicate depleted iron stores, and at that point many people already experience fatigue and reduced exercise tolerance. Your muscles rely on iron to transport oxygen and clear waste products; without enough, even routine physical activity can leave you sore and exhausted.
Severe iron deficiency goes further, weakening both skeletal and cardiac muscle by increasing oxidative stress inside cells. That means your muscles are less efficient and more prone to fatigue, and your heart has to work harder to compensate. The result is a person who feels heavy, achy, and short of breath doing things that used to feel easy. Women with heavy periods, vegetarians, and frequent blood donors are at the highest risk.
Vitamin D deficiency produces a similar picture. Roughly a quarter of older adults in the U.S. have insufficient vitamin D levels, and low levels are strongly linked to fatigue and diffuse musculoskeletal pain. A simple blood test can check both iron and vitamin D, and correcting a deficiency often produces noticeable improvement within weeks.
Thyroid Problems
Your thyroid gland controls the rate at which your cells produce energy. When it’s underactive, a condition called hypothyroidism, nearly every system in the body slows down. Thyroid hormone is a primary regulator of skeletal muscle function, governing how mitochondria produce energy and how muscles contract. A deficiency causes your fast-twitch muscle fibers to atrophy and shift toward slower fiber types, which is why people with hypothyroidism often describe feeling sluggish, weak, and stiff.
Beyond muscle symptoms, hypothyroidism typically causes weight gain, cold sensitivity, constipation, dry skin, and brain fog. The fatigue tends to be persistent and unresponsive to rest. Diagnosis involves a blood test measuring thyroid-stimulating hormone (TSH) and thyroid hormone levels. Once treatment begins, most people see improvement within about two months as hormone levels normalize.
Fibromyalgia
If you’ve had widespread pain and fatigue for three months or more without another explanation, fibromyalgia is a possibility. It’s diagnosed based on how many body areas are affected and the severity of accompanying symptoms like unrefreshing sleep, cognitive difficulties, and fatigue. There’s no blood test or imaging scan that confirms it. Instead, clinicians use a scoring system that accounts for the number of painful areas across your body and the overall burden of symptoms. The condition correctly captures about 88% of cases using this approach.
Fibromyalgia pain tends to move around and fluctuate in intensity. Many people describe it as a deep, persistent aching rather than sharp or localized pain. The fatigue is often as disabling as the pain itself, and it doesn’t improve much with sleep. The condition is more common in women and frequently overlaps with anxiety, depression, and irritable bowel syndrome.
Autoimmune Conditions
Rheumatoid arthritis is one of the more recognizable autoimmune causes of fatigue and body aches. Early signs include tenderness or pain in small joints like the fingers and toes, though larger joints like the knees and shoulders can be affected too. A hallmark feature is symmetry: the same joints on both sides of your body tend to hurt. The fatigue in rheumatoid arthritis isn’t just from poor sleep or dealing with pain. It’s driven by chronic inflammation, measurable through blood markers like C-reactive protein and sedimentation rate.
Lupus, Sjögren’s syndrome, and other autoimmune diseases also cause fatigue and aching through similar inflammatory pathways. These conditions tend to come with additional symptoms, such as rashes, dry eyes, or joint swelling, that help distinguish them from other causes.
Chronic Fatigue Syndrome (ME/CFS)
ME/CFS is a distinct condition defined by fatigue that lasts longer than six months, is not lifelong, and is not substantially relieved by rest. The fatigue must be severe enough to significantly reduce your ability to function at work, school, or in daily life compared to before you got sick. Two other required features are post-exertional malaise, where symptoms worsen after physical or mental effort that previously would have been fine, and unrefreshing sleep, where a full night’s rest doesn’t make you feel restored.
In addition to those three core symptoms, a diagnosis requires at least one of the following: cognitive impairment (problems with memory, focus, or processing speed) or orthostatic intolerance (symptoms that worsen when you stand up). ME/CFS often begins after a viral infection and can be profoundly disabling. It is not the same as being chronically tired, and the post-exertional crashes are its most distinguishing feature.
Poor Sleep and Sleep Apnea
Sleep that’s too short or too fragmented leaves you fatigued, but it also increases pain sensitivity. Obstructive sleep apnea creates a particularly strong link between the two symptoms. During apnea episodes, oxygen levels drop repeatedly throughout the night. This intermittent oxygen deprivation triggers inflammatory pathways that sensitize pain receptors, making you more aware of aches and discomfort during the day. Research has shown that nocturnal oxygen drops are independently associated with increased pain, separate from the effects of fragmented sleep alone.
Signs of sleep apnea include loud snoring, gasping during sleep, morning headaches, and excessive daytime sleepiness. Many people don’t realize they have it until a partner notices the breathing pauses. It’s more common in people who are overweight, but it can affect anyone.
Medication Side Effects
Statins, the cholesterol-lowering drugs taken by over 40 million Americans, are one of the most common medication-related causes of muscle aches and fatigue. In clinical practice, up to 20% of statin users report muscle symptoms, though the rate in controlled trials is lower, around 3 to 6%. The discrepancy likely reflects the broader range of people taking statins in the real world compared to carefully selected trial participants. For most people the symptoms are mild and reversible with a dosage change or switch to a different statin, but a small percentage develop more severe muscle problems.
Other medications that commonly cause fatigue and body aches include blood pressure drugs, antihistamines, antidepressants, and certain antibiotics. If your symptoms started or worsened after beginning a new medication, that timing is worth noting.
How to Narrow Down the Cause
The most useful question is how long your symptoms have lasted. Fatigue and aches that started within the past week or two, especially with a fever, point toward infection. Symptoms lasting weeks to months without improvement suggest a deficiency, hormonal issue, or chronic condition. Pay attention to what else is happening: joint swelling suggests autoimmune disease, weight gain and cold intolerance suggest thyroid problems, and crashes after exertion suggest ME/CFS.
A basic workup typically includes blood tests for thyroid function, iron and ferritin levels, vitamin D, inflammatory markers, and a complete blood count. These are inexpensive, widely available, and can rule in or rule out several of the most treatable causes in a single visit. If those come back normal and symptoms persist, further evaluation for conditions like fibromyalgia, sleep apnea, or autoimmune disease becomes the next step.