Why am I always tired? 7 possible causes
Seven of the most common causes of persistent fatigue we see in internal medicine — and how to know when it's time to get it checked out.
If you've been searching “why am I always tired,” you're far from alone. It's one of the most common health questions people ask, and one of the most common reasons patients come to internal medicine in the first place. Fatigue can be frustrating precisely because it's vague: it's not a single symptom pointing to a single cause. It's your body's way of telling you something is off — and the “something” could be anywhere from your sleep schedule to your thyroid.
Here are seven of the most common causes we see, and how to know when it's time to get it checked out.
Poor sleep quality (not just poor sleep quantity)
Getting eight hours doesn't guarantee restorative sleep. Sleep apnea, frequent waking, and even too much screen time before bed can leave you technically “rested” but functionally exhausted. If you snore, wake up gasping, or still feel tired after a full night's sleep, that's worth flagging to your doctor — sleep apnea in particular is common, treatable, and often missed.
Iron deficiency or anemia
Low iron is one of the most common — and most overlooked — causes of fatigue, especially in women. Anemia reduces how much oxygen your blood can carry to your muscles and brain, which shows up as tiredness, brain fog, and sometimes shortness of breath. A simple blood test can confirm it.
Thyroid dysfunction
An underactive thyroid (hypothyroidism) slows down nearly every system in your body, and fatigue is often the first sign — sometimes before weight changes, hair thinning, or feeling cold all the time show up. Thyroid issues are easy to test for and, in most cases, straightforward to manage once identified.
Vitamin D or B12 deficiency
Both are common, both are easy to miss, and both can leave you feeling drained even when everything else checks out. Vitamin D deficiency in particular is widespread and often asymptomatic aside from fatigue.
Blood sugar swings or undiagnosed diabetes
Constant fatigue — especially after meals — can be an early sign of insulin resistance or diabetes before other symptoms appear. If fatigue comes with increased thirst, frequent urination, or unexplained weight changes, it's worth ruling out.
Chronic stress, anxiety, or low mood
Mental and physical health aren't separate systems — chronic stress and mood changes are a well-documented cause of persistent fatigue, sometimes independent of how much sleep you're getting. This doesn't mean “it's all in your head”; it means your nervous system is working overtime, and that has a physical cost.
Medication side effects
Certain blood pressure medications, antihistamines, and other common prescriptions list fatigue as a side effect. If your tiredness started around the same time as a new medication or dose change, that timing is worth mentioning at your next visit.
When fatigue is a sign of something more urgent
Most fatigue is manageable and explainable. But see a doctor sooner rather than later if fatigue comes with:
- Chest pain, shortness of breath, or heart palpitations
- Unexplained weight loss
- Fever or night sweats
- Fatigue that's severe, sudden, or getting worse week over week
How telehealth can help you figure out the cause
You don't need to guess which of these seven applies to you, and you don't need an in-person visit to start finding out. A telehealth internal medicine visit can review your symptoms, history, and medications, and order the right bloodwork to narrow things down — all without taking a morning off work or sitting in a waiting room.