Fatigue is one of the most frequent reasons for seeking medical advice: you may feel drained, inefficient, irritable, unable to keep up… and sometimes even tired despite rest . When it's occasional, it's often a normal signal from the body. But when it persists, intensifies, or is accompanied by unusual symptoms, fatigue can become asthenia and reveal an imbalance (sleep problems, stress, deficiency, infection, hormonal disorder, chronic illness…).

In this guide, you will understand the main causes of fatigue , learn to spot the warning signs , and above all apply a concrete action plan for lasting recovery.

“Normal” fatigue or asthenia: knowing the difference

Fatigue after physical or mental exertion, which disappears with rest, is generally considered "physiological." Asthenia, on the other hand, refers abnormal fatigue that persists despite sleep and rest , accompanied by a distressing feeling of being "unable" to manage daily life.

Typical signs of abnormal fatigue

• Fatigue present from the morning, feeling like I'm not "recharging"
• Decreased concentration, memory, attention
• Decreased motivation, irritability, hypersensitivity
• Decreased physical performance
• A feeling of imbalance between what you should do and what you can do

At what point do we start talking about chronic fatigue?

Chronic asthenia is defined as symptoms that persist for more than 6 months .

The main causes of fatigue (the most frequent in practice)

Fatigue often results from a combination of factors: lifestyle , mental health , and medical causes .

1) Lack of sleep, non-restorative sleep, disrupted sleep rhythm

Typical causes:

• sleep debt (late bedtimes, nighttime awakenings)
• recurring insomnia
• irregular hours / jet lag
• awakenings related to a child, etc.

Expert point : “Sleeping 8 hours” is not enough if sleep is fragmented (micro-awakenings), if wake-up time varies too much, or if there is a sleep disorder (e.g., sleep apnea ).

2) Stress, overwork, burnout, life difficulties

Overwork, professional stress, burnout (professional exhaustion) or personal difficulties (breakup, isolation…) are major causes of reactive fatigue.

Common associated signs (useful for orientation):

• emotional disturbances (anxiety, sadness, irritability)
• Cognitive disorders (attention, memory)
• physical manifestations (headaches, pain, dizziness, digestive problems)

3) Infections and convalescence (including long Covid)

Viral/bacterial infections (flu, mononucleosis, Covid-19, etc.) often cause fatigue that improves with recovery.

Long Covid : fatigue can persist beyond several weeks, be fluctuating, with very long recovery after exertion (post-exertional malaise).

4) Deficiencies and “blood” causes: anemia (including iron deficiency)

All anemias can cause fatigue , often marked when the anemia is severe, and of rapid onset.

5) Endocrine causes (thyroid, diabetes, adrenal glands…)

Endocrine diseases such as hypothyroidism or type 2 diabetes may be involved, especially if the treatment is inadequate.

6) Sleep disorders (apnea, restless legs syndrome…)

Non-compensatory sleep may be related to:

• sleep apnea
• restless legs syndrome
• insomnia, parasomnia
• narcolepsy/hypersomnia

7) Medications, alcohol, substances, poisoning

Certain medications (psychotropics, sedatives, antidepressants, diuretics, antihistamines, etc.), alcohol, substances or certain intoxications (e.g., carbon monoxide) can cause fatigue .

When fatigue should raise a red flag: the “red flags” not to ignore

Consult a doctor promptly if fatigue is present:

• brutal, intense, unusual without explanation
• associated with shortness of breath , chest pain, palpitations, and malaise
• associated with prolonged fever , night sweats, unintentional weight loss
• associated with neurological disorders (weakness in a limb, speech disorders, vision problems)
• associated with profound sadness , dark thoughts, and an inability to function in daily life
• persistent despite rest, or lasting (weeks to months), especially if it worsens

Useful reference point : fatigue that persists despite rest corresponds to abnormal fatigue ( asthenia ) and deserves evaluation.

Expert action plan: what to do concretely against fatigue (7 levers)

Objective: to achieve “quick gains” (7–10 days) + a sustainable strategy (4–8 weeks).

1) Measure before you act: your mini-assessment in 5 minutes

For 7 days, note:

• bedtime/wake-up time + nighttime awakenings
• energy level (0–10) morning / afternoon / evening
• caffeine (amount + time), alcohol
• physical activity (duration, intensity)
• Screen time in the evening (duration + time)

Why: fatigue is often multifactorial; without tracking, we correct the wrong lever.

2) Sleep: the routine that most improves fatigue

Highly effective actions:

1. Fixed wake-up time (same weekend) → stabilizes the biological clock.
2. Daylight in the morning for 10–20 minutes (outdoors if possible).
3. Screens : reduce late-night exposure.
4. Caffeine : avoid after early afternoon if you are sensitive.
5. If snoring + daytime sleepiness: suspect a sleep disorder (e.g., apnea ) and talk to a doctor.

Common mistake : “compensating” by sleeping very late on weekends → this maintains a shift and fatigue of the social jet lag type.

3) Physical activity: neither too much nor zero

Fatigue can occur after intense exercise, but also in cases of sedentary lifestyle ( muscle loss).

Recommended (simple) approach:

• 3–5 days/week: 20–40 min moderate (brisk walking, gentle cycling)
• 2 days/week: light strengthening (20 min)
• If post-viral/long Covid fatigue: progress very gradually, avoid the “no pain no gain” approach (risk of over-fatigue)

4) Nutrition & hydration: stabilizing energy

The goal is to avoid the roller coaster effect (reactive hypoglycemia, snacking).

• Protein at every meal (satiety + stability)
• Fiber + complex carbohydrates (legumes, whole grains)
• Strategic snack if you're feeling sluggish (yogurt/cottage cheese + fruit, a handful of nuts)
• If anemia / deficiency is suspected: do not self-supplement “at random” → have the cause checked (and treated)

5) Stress: reduce mental load instead of adding "to-do" items

If fatigue is linked to stress/overwork/burnout, the goal is not to "better organize" but to reduce the strain :

• Prioritize 1–3 essential tasks per day
• blocks without notifications
• short scheduled breaks
• If you experience signs of burnout (emotional/cognitive/physical), consult a professional and consider adjusting your work schedule or stopping work if necessary.

6) Medications and substances: check for fatigue-related effects

Fatigue can be linked to medication (sedatives, psychotropic drugs, antihistamines…), alcohol or other substances: never stop on your own, but report the fatigue to the prescriber for adjustment.

7) When and how to consult effectively (to save time)

Arrive with:

• your 7-day monitoring (sleep/energy)
• List of medications/supplements
• associated symptoms (weight loss, fever, shortness of breath, heavy periods, snoring, mood swings…)
• Context: recent infection, stress, job change, postpartum, etc.

Useful reference point : 10 to 25% of people consulting a general practitioner complain of being “always tired”.

Quick overview: probable causes ↔ clues ↔ first action

FAQ 

Why am I always tired?

Most often, it is multifactorial: lack of sleep or non-restorative sleep, stress/overwork, convalescence, deficiency (e.g., anemia ), sleep disorder (apnea), or hormonal cause (thyroid).

Fatigue: when should you consult a doctor?

When it persists despite rest, lasts several weeks, worsens, or is accompanied by warning signs (prolonged fever, weight loss, shortness of breath, chest pain, neurological disorders, significant psychological distress).

What is asthenia?

Asthenia is an abnormal fatigue that does not disappear (or hardly disappears) with rest a perceived inability to carry out daily activities.

At what point do we start talking about chronic fatigue?

When symptoms persist for more than 6 months ( chronic asthenia ).

Can fatigue be caused by burnout?

Yes. Burnout combines progressive fatigue with emotional (anxiety, sadness, irritability), cognitive (attention, memory) and physical (pain, digestive problems, dizziness…) symptoms.

Why am I tired after an infection (or after Covid)?

After an infection, fatigue is common during convalescence. In the case of long Covid , it can persist for more than several weeks, fluctuate, and be accompanied by a very long recovery period after exertion.

Conclusion

Fatigue isn't "all in your head": it's a common, often multifactorial symptom that can be normal... or become asthenia it persists despite rest. By implementing a simple plan (7-day tracking, sleep routine, gradual return to activity, stress reduction, medication review), you can already see significant improvements. If fatigue persists, worsens, or is accompanied by warning signs, the most effective step is a structured consultation with your tracking data to identify the cause and treat it at its source.