Lack of energy usually comes from something identifiable: a nutritional gap, poor sleep, a hormonal shift, or a mental health condition. In many cases, multiple factors overlap. The good news is that most causes are treatable once you know what to look for.
Iron and Vitamin Deficiencies
Iron deficiency is one of the most common nutritional causes of persistent fatigue. Your body needs iron to make hemoglobin, the protein in red blood cells that carries oxygen to every tissue. When iron runs low, your cells literally get less oxygen, which shows up as tiredness, shortness of breath during normal activity, and difficulty concentrating. Women with heavy periods, vegetarians, and frequent blood donors are at higher risk.
Vitamin B12 plays a similarly critical role. It’s required for healthy red blood cell formation and nervous system function. When you’re deficient, your body produces abnormally large red blood cells that don’t work efficiently, a condition called megaloblastic anemia. The result is deep fatigue that doesn’t improve with rest. B12 deficiency is especially common in older adults and people who eat little or no animal products, since meat, fish, and dairy are the primary dietary sources.
Magnesium is another overlooked nutrient. Every cell in your body uses a molecule called ATP as its energy currency, and ATP can only function when it’s bound to magnesium. Without enough magnesium, your cells can’t efficiently use the energy they produce. Muscle cramps, poor sleep, and low energy are classic signs of inadequate magnesium intake.
Blood Sugar and Energy Crashes
If your energy tanks an hour or two after meals, your blood sugar response is a likely culprit. When you eat a high-sugar or refined-carb meal, your blood sugar spikes rapidly. Your body responds by releasing a large burst of insulin to bring it back down, but it often overcorrects. The result, called reactive hypoglycemia, is a sharp drop in blood sugar that triggers tiredness, brain fog, and cravings for more sugar. This cycle can repeat multiple times a day if your diet leans heavily on processed carbohydrates, sweetened drinks, or white bread and pasta.
Eating meals that combine protein, healthy fat, and fiber slows down sugar absorption and produces a more stable energy curve. The difference can be dramatic, even within a few days of changing what you eat.
Thyroid Problems
Your thyroid gland controls your metabolism, essentially setting the speed at which your body burns fuel. When the thyroid underperforms (hypothyroidism), your metabolism slows down, which can make you feel exhausted all the time, gain weight without eating more, and feel cold when others are comfortable.
There’s also a milder form called subclinical hypothyroidism, where thyroid hormone levels test in the normal range but the signal from your brain telling the thyroid to work harder (TSH) is slightly elevated. Even this mild version can cause noticeable fatigue. Hypothyroidism is far more common in women and becomes more likely after age 40. A simple blood test can detect it.
Depression and Mental Health
Depression doesn’t just affect your mood. It physically drains your energy. The brain chemicals that regulate motivation and movement, particularly dopamine and norepinephrine, become depleted in depression. Research published in the International Journal of Neuropsychopharmacology identified that dopamine depletion in a specific brain reward center reduces the likelihood of exerting effort for any activity, which may explain the profound lack of motivation and physical sluggishness that depression causes.
This type of fatigue feels different from being tired after a long day. It’s a heaviness in the body, a sense that even small tasks require enormous effort. People often describe it as feeling like they’re moving through mud. If your low energy comes alongside persistent sadness, loss of interest in things you used to enjoy, or changes in appetite and sleep, depression is worth considering seriously.
Anxiety also burns through energy, though in a different way. Your body stays in a low-grade fight-or-flight state, which is physically exhausting over time even if you’re sitting still.
Sleep Quality vs. Sleep Quantity
Getting seven or eight hours in bed doesn’t guarantee restorative sleep. Sleep apnea, a condition where your airway partially collapses during sleep, fragments your rest dozens or hundreds of times per night without you realizing it. You wake up feeling like you barely slept. Snoring, waking with a dry mouth, and morning headaches are telltale signs.
Alcohol is another common sleep disruptor. It helps you fall asleep faster but suppresses the deep, restorative stages of sleep later in the night. Even two drinks in the evening can leave you feeling unrested the next morning. The same goes for screens before bed: the blue light delays your body’s natural sleep hormone release, pushing back the onset of quality sleep even if you’re lying in bed at the same time.
Dehydration
Mild dehydration, losing just 1 to 2 percent of your body weight in fluid, is enough to impair cognitive performance, slow your reaction time, and make you feel sluggish. For a 150-pound person, that’s only about 1.5 to 3 pounds of water loss. Most people don’t register this level of dehydration as thirst. They register it as fatigue, difficulty focusing, or a mild headache. Coffee and tea count toward your fluid intake, but they don’t fully replace plain water, especially in hot weather or after exercise.
Sedentary Lifestyle
It sounds counterintuitive, but using less energy makes you feel like you have less energy. Regular physical activity improves how efficiently your mitochondria (the power plants inside your cells) produce energy. When you’re sedentary for long stretches, that system downregulates. Your body adapts to low demand by producing less capacity.
Even moderate activity, like a 20-minute walk, increases blood flow, stimulates alertness chemicals in the brain, and improves sleep quality that night. People who start a consistent exercise routine typically report noticeable improvements in energy within two to three weeks, even before any changes in fitness show up on a test.
Chronic Fatigue Syndrome
When fatigue is severe, lasts longer than six months, and doesn’t improve with rest, it may be myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). This is a distinct medical condition, not just “being tired.” The CDC’s diagnostic criteria require a substantial reduction in your ability to do things you could do before, fatigue that is new (not lifelong) and not caused by unusual exertion, plus unrefreshing sleep, meaning a full night’s sleep doesn’t make you feel better.
The hallmark symptom is post-exertional malaise: a worsening of symptoms after physical, mental, or even emotional effort that wouldn’t have been a problem before. This crash typically hits 12 to 48 hours after the activity and can last days or weeks. Patients must also have either cognitive impairment (problems with memory, focus, and information processing) or orthostatic intolerance (symptoms that worsen when standing or sitting upright). These symptoms need to be present at least half the time and at moderate or greater intensity.
Research has consistently found low levels of coenzyme Q10 in people with chronic fatigue, pointing to problems with how mitochondria produce energy at the cellular level. ME/CFS remains difficult to diagnose because there’s no single lab test for it, but awareness among doctors has improved significantly in recent years.
Other Medical Causes Worth Checking
Several other conditions commonly cause persistent low energy. Diabetes and prediabetes impair your cells’ ability to use glucose for fuel, leaving you tired even when blood sugar is technically available. Anemia from causes other than iron deficiency, including chronic kidney disease and certain autoimmune conditions, reduces oxygen delivery the same way. Heart conditions that limit how efficiently blood circulates can make even light activity feel draining.
Certain medications are notorious energy thieves. Antihistamines, blood pressure medications, and some antidepressants list fatigue as a common side effect. If your low energy started around the time you began a new medication, that connection is worth raising with whoever prescribed it.
Infections, both acute and lingering, are another trigger. Post-viral fatigue after illnesses like COVID, mononucleosis, or the flu can persist for weeks or months after the infection itself has cleared. The mechanism overlaps with what’s seen in ME/CFS, involving immune system activation and mitochondrial stress.