Brief Answer:
investigation is required

Detailed Answer:
Hello,

there are various potential causes, so I'll make a short list with some common disorders and how they can be documented.

- heart failure: the EKG is usually abnormal but this is not the way to diagnose heart failure. An ultrasound scan of the heart (echo) is required to evaluate the heart functionality. A cardiologist should do the echo.
- chronic obstructive pulmonary disease (COPD): smoking usually causes COPD. COPD may present with cough, phlegm, shortness of breath and wheezing. A pulmonologist can diagnose COPD based on clinical information and spirometry.
- anemia: low hematocrit may cause fatigue, sleepiness, headaches, difficulty to concentrate, etc. A simple blood test is enough to diagnose it.
- sleep apnea: it may cause sleepiness and fatigue and may event affect the heart. If you snore, you're fat and smoke then the likelihood for sleep apnea is higher. An ENT and pulmonologist should have a look at you before you do the sleep-apnea test (sleep-study).
- thyroid disorders: a high TSH value (blood test) can be related to fatigue and sleepiness. This is called hypothyroidism.

I hope you find my comments helpful!
Kind regards!

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