You’re not lazy. You’re tired, again, by 3 p.m., even on days you didn’t train. If you’re always tired after 35, it’s tempting to blame age, work or kids and push through with more coffee. Sometimes that’s the whole story. Often it isn’t, and the cause is something you can identify and fix.
Key takeaways
- Fatigue is one of the most common reasons adults see a doctor, and many causes are lifestyle-related or treatable once identified.
- The usual suspects: short sleep, sleep apnea, low iron or B12, low vitamin D, thyroid problems, blood sugar issues, alcohol, under-eating or over-training, stress and low mood, medications and too little activity.
- Start with the basics (sleep, alcohol, food, movement) for two weeks, then see a doctor if you are still exhausted.
- Don’t self-treat with iron or high-dose supplements. Get tested first.
- Fatigue with chest pain, shortness of breath, feeling faint or unusual bleeding needs emergency care; fatigue with weight loss or mood changes needs a doctor’s visit soon.
First, how common is this?
Very. A 2023 review in American Family Physician notes that fatigue is among the top ten reasons people visit primary care doctors. Fatigue lasting six months or longer is classed as chronic. The same review points out that testing everyone for everything is low-yield. Doctors get much further by asking good questions first, so the more specific you can be about your symptoms, the better.
Below are ten common causes in men 35 and over, what typically gives each one away, and what to do.
Always tired after 35: 10 common causes
1. Plain sleep debt
Sleep experts recommend that adults get at least 7 hours a night on a regular basis, and the CDC reports that more than a third of American adults fall short. Six hours a night feels “normal” after a while, but the deficit accumulates. Lab studies show that even about a week of short sleep can lower testosterone in healthy men.
What to do: Track your actual sleep for a week. If you’re under 7 hours most nights, fix that before anything else. Our guide on how to sleep better after 35 has a step-by-step plan.
2. Sleep apnea
If you sleep 7 to 8 hours and still wake up exhausted, think about obstructive sleep apnea. Your airway repeatedly narrows or closes during sleep, fragmenting it without you knowing. The NHS lists loud snoring, gasping or choking noises, breathing that stops and starts, morning headaches and heavy daytime sleepiness as key signs. Risk rises with weight, a large neck, alcohol, smoking and age.
What to do: Ask your partner what they hear at night. If the signs fit, see your doctor about a sleep study. Don’t drive if you’re struggling to stay awake.
3. Low iron (iron deficiency anaemia)
Iron deficiency anaemia causes tiredness, shortness of breath, palpitations and pale skin. In men, it deserves attention: the most common cause in men is bleeding somewhere in the stomach or intestines, which can come from ulcers, regular use of anti-inflammatory painkillers (NSAIDs) or, less commonly, bowel or stomach cancer.
What to do: Get a blood test (full blood count and iron studies). Don’t take iron supplements on a hunch. Too much iron can be harmful, and a supplement could hide a bleed that needs investigating. Vegetarians should plan iron-rich foods (lentils, chickpeas, tofu, fortified cereals, leafy greens) with a vitamin C source to improve absorption.
4. Low vitamin B12
The NHS lists extreme tiredness, low energy, pins and needles, a sore red tongue, muscle weakness, memory problems and low mood as symptoms of B12 deficiency, and some can appear before anaemia develops. Common causes include pernicious anaemia (an autoimmune condition that blocks absorption), a vegan or very restrictive diet without fortified foods or supplements, and medicines such as proton pump inhibitors (acid-reducers). Long-term metformin use is also linked to lower B12.
What to do: If you’re vegetarian, vegan, take acid-reducers or metformin, or have tingling in your hands or feet, ask for a B12 test. Read more in our guide to vitamin B12 for vegetarians.
5. Low vitamin D
Mayo Clinic includes low vitamin D among the conditions associated with fatigue. Men who work indoors, live far from the equator, have darker skin or carry extra body fat are more likely to run low.
What to do: Ask about a blood test if you’re in a risk group, rather than guessing at a dose. Our guide to vitamin D for men over 35 covers testing, food sources and safe intakes.
6. Thyroid problems
Both an underactive and an overactive thyroid can leave you exhausted. An underactive thyroid tends to come with feeling cold, weight gain, constipation and sluggishness. The NHS links an overactive thyroid to tiredness with restlessness, anxiety, irritability and muscle weakness.
What to do: A simple blood test (TSH, sometimes with thyroid hormones) is a standard part of a fatigue check-up.
7. Blood sugar problems
Tiredness together with increased thirst, frequent urination (especially at night) and unexplained weight loss can be signs of diabetes. Risk of type 2 diabetes rises with age, waist size and family history, and it often develops quietly over years.
What to do: Ask for a fasting glucose or HbA1c test, especially if you carry weight around the middle or have a family history. Strength training and losing visceral fat both help; see our guide on how to lose fat and keep muscle after 35.
8. Alcohol
Alcohol use is on Mayo Clinic’s list of lifestyle causes of fatigue. A review of 20 sleep-lab studies found that while alcohol helps you fall asleep faster, it disrupts sleep in the second half of the night and, at moderate and higher doses, reduces REM sleep. A few weeknight drinks can quietly cost you a lot of sleep quality.
What to do: Try two to four alcohol-free weeks and notice how you feel. If cutting back is harder than you expected, talk to your doctor.
9. Under-eating and over-training
Mayo Clinic lists both too little and too much physical activity, along with poor eating habits, as causes of fatigue. Men who combine an aggressive diet with hard training several times a week are at risk of low energy availability, meaning the body doesn’t get enough fuel for both training and normal function. The International Olympic Committee recognizes this as a problem in male as well as female athletes (called Relative Energy Deficiency in Sport, or REDs).
What to do: Watch for stalled strength, poor sleep, irritability, low libido and frequent minor illnesses. Eat enough protein (see our guide to protein after 35), avoid very large calorie deficits, and build in easy weeks.
10. Stress, low mood and anxiety
The NHS lists stress, depression and life challenges such as bereavement or caring for a new baby among the common causes of tiredness. Low mood doesn’t always look like sadness; it can show up as tiredness, poor sleep, irritability and losing interest in things you used to enjoy.
What to do: Be honest with yourself and your doctor. Effective treatments exist, including talking therapies, and exercise helps too. If you have thoughts of harming yourself, seek emergency help right away.
Two more to keep on your radar
Medications
Mayo Clinic notes that allergy and cough medicines, pain medicines, heart medicines and antidepressants can all cause fatigue. If your tiredness started after a new prescription or a dose change, tell your doctor. Never stop a prescribed medicine on your own.
Too little activity
It sounds backwards, but sitting all day can make you more tired. A University of Georgia analysis of 70 randomized trials with more than 6,800 participants found that sedentary people who started regular exercise reported more energy and less fatigue than those who didn’t, a finding the researchers described as very consistent.
What to do: Start small: daily walks, then two or three short strength sessions a week. Our guide to starting to work out after 40 can help.
Cause, clues and what to check
| Possible cause | Typical clues | What to check or try |
|---|---|---|
| Sleep debt | Under 7 h most nights, long weekend lie-ins | Sleep diary for 1–2 weeks; fixed wake time |
| Sleep apnea | Loud snoring, gasping, morning headaches, sleepy despite enough hours | Doctor review; sleep study |
| Low iron | Breathlessness, palpitations, pale skin | Full blood count, iron studies; find the cause |
| Low B12 | Pins and needles, sore tongue, memory issues; vegetarian, acid-reducers, metformin | B12 blood test |
| Low vitamin D | Little sun, indoor work, darker skin, higher body fat | 25(OH)D blood test if at risk |
| Thyroid | Feeling cold, weight gain, constipation (low); restless, anxious (high) | TSH blood test |
| Blood sugar | Thirst, frequent urination, weight loss, family history | Fasting glucose or HbA1c |
| Alcohol | Regular evening drinks, poor second half of night | 2–4 alcohol-free weeks |
| Under-eating / over-training | Stalled strength, low libido, irritability, frequent colds | Review calories, protein and training load |
| Stress / low mood | Poor sleep, irritability, loss of interest | Talk to your doctor; screening for depression or anxiety |
| Medications | Fatigue began after a new drug or dose change | Medication review with your doctor |
| Low activity | Mostly seated day, few steps | Daily walks, regular strength training |
When to see a doctor
Mayo Clinic advises booking an appointment if fatigue lasts two weeks or more despite rest, less stress, a healthy diet and enough fluids. The NHS also recommends seeing a GP if you’ve felt tired for a few weeks without knowing why, if it’s affecting your daily life, if it comes with other symptoms such as weight loss or mood changes, or if someone has noticed you gasping or choking in your sleep.
Your doctor will usually ask about sleep, mood, alcohol, medicines and other symptoms, examine you, and may order basic blood tests such as a full blood count, metabolic panel and thyroid test. Arrive with a short list of your symptoms, when they started, what you’ve already tried and all the medicines and supplements you take.
Safety note: Get emergency help right away if fatigue comes with chest pain, shortness of breath, a fast or irregular heartbeat, feeling like you might pass out, severe headache, severe abdominal, pelvic or back pain, or unusual bleeding (including blood in stool or vomit). If you have thoughts of harming yourself, call your local emergency number. Don’t start iron, high-dose vitamins or other supplements to treat fatigue without testing and talking to your doctor, as some can cause harm or interact with medicines.
A simple two-week reset
- Sleep: fixed wake time every day and a 7+ hour sleep window.
- Alcohol: none on weeknights.
- Caffeine: nothing after early afternoon.
- Food: three regular meals with protein, vegetables and whole grains; no crash dieting.
- Movement: a daily 20–30 minute walk; keep strength sessions moderate.
- Notes: rate your energy 1–10 each day.
If your energy scores haven’t improved after two weeks, take your notes to your doctor. That diary makes the appointment far more useful.
FAQ
Is it normal to feel more tired after 35?
Many men notice they recover more slowly from short sleep and hard training as they get older. But persistent, unexplained exhaustion is not something to accept as “just age.” It’s worth checking.
What blood tests should I ask for?
Your doctor will choose based on your symptoms, but common ones include a full blood count, iron studies, B12, thyroid function, blood glucose or HbA1c, and sometimes vitamin D. Testing without a reason is often low-yield, so describe your symptoms clearly.
Can low testosterone cause fatigue?
It can be one factor, but the causes above, especially sleep, weight and alcohol, are far more common and also affect testosterone. See our guide to testosterone after 35 and discuss testing with your doctor rather than starting any treatment on your own.
Should I take an energy supplement?
Not before you know the cause. Fixing a real deficiency with your doctor helps; taking random “energy” products usually doesn’t, and some contain large amounts of caffeine.
Could it be chronic fatigue syndrome?
ME/CFS involves extreme fatigue lasting at least three months, often with memory problems and flu-like symptoms. It’s much less common than the causes above and is diagnosed by a doctor after other causes are ruled out.
Free tools: plan your sleep with the sleep calculator and find your biggest energy drains with the Fit After 35 score.
Sources
- American Family Physician (2023): Fatigue in adults: evaluation and management
- Mayo Clinic: Fatigue causes
- Mayo Clinic: Fatigue, when to see a doctor
- NHS: Tiredness and fatigue
- NHS: Sleep apnoea
- NI Direct (NHS information): Iron deficiency anaemia
- NHS: Vitamin B12 or folate deficiency anaemia
- AASM and Sleep Research Society: Recommended amount of sleep for a healthy adult
- CDC: 1 in 3 adults don’t get enough sleep
- Live Science: Review of alcohol’s effects on sleep (Ebrahim et al., 2013)
- ScienceDaily: Exercise reduces fatigue in sedentary people (Puetz et al., 2006)
- 2023 IOC consensus statement on Relative Energy Deficiency in Sport (REDs)
This article is for general education and is not medical advice. Check with your doctor before starting a new training or nutrition plan, especially if you have a health condition.
