Photo by Adrian Swancar on Unsplash
Feeling constantly tired can be frustrating, especially when you’re doing everything “right”—getting enough sleep, eating well, and managing your lifestyle. But what if the real reasons behind your fatigue lie deeper in your biology? I’m Dr. Adrian Laurence, a family physician with over 17 years of experience, and I want to share with you five key biological reasons why you might always feel tired after 35—and, importantly, what you can do about each one.
1. Declining Sleep Quality: What Happens to Your Brain After 35?
Most people assume that feeling tired as you age is just normal, but there’s a specific brain change that happens around age 35 that nobody talks about. It’s not about how many hours you sleep—it’s about the quality of that sleep.
Our sleep has different stages, and the most important one for restoration is called slow wave sleep—the deep sleep phase where your brain clears out waste, consolidates memories, and recharges your energy. Unfortunately, research shows that this deep sleep decreases by about 2% per decade after 35, so by age 55, you’re getting roughly 40% less restorative sleep than you did in your twenties.
Alongside this, your circadian rhythm—your body’s internal clock—starts to weaken. This clock controls when you feel sleepy and awake by regulating hormones like melatonin and cortisol. When it falters, you experience more fragmented sleep with frequent brief awakenings you might not even remember, which pulls you out of deep sleep phases.
Another subtle but crucial change is in how your body temperature is regulated during sleep. Normally, your core body temperature drops at night to help trigger deep sleep, but this drop becomes less pronounced with age, leading to lighter, less restorative sleep.
How to Fix It
- Cognitive Behavioral Therapy for Insomnia (CBTI): This is the gold standard treatment for improving sleep quality without medication. Studies show it’s more effective than sleeping pills for long-term improvement.
- Strategic Light Exposure: Get bright light exposure within 30 minutes of waking to reset your circadian rhythm.
- Limit Screen Time: Avoid screens at least two hours before bedtime to prevent melatonin disruption.
- Optimize Bedroom Temperature: Keep your room between 65-68°F (18-20°C) to help your core body temperature drop.
For more on sleep architecture and improving sleep quality, check out this review on aging and sleep.
2. Hormonal Shifts: The Hidden Energy Drainers
After 40, hormonal changes become a major cause of fatigue in both men and women, though the patterns differ.
Women: Progesterone, Estrogen, and Menopause
For women, declining progesterone is a key issue. Progesterone not only supports reproduction but is also a natural sleep promoter. When levels drop during menopause, it becomes harder to enter deep sleep stages. Estrogen fluctuations add to this, creating unpredictable sleep disruptions.
One of the biggest culprits is vasomotor symptoms—hot flashes and night sweats that wake women multiple times a night, severely impacting restorative sleep. Studies show that 35-60% of postmenopausal women experience sleep disorders, a stark increase compared to premenopausal rates.
Men: Testosterone Decline
Men experience a natural decline in testosterone of about 1-2% per year starting around age 30. But recent studies show men today have much lower testosterone than those decades ago, leading to a condition called late onset hypogonadism when levels drop below 300 ng/dL.
Low testosterone causes lethargy, poor sleep, muscle loss, and increased belly fat. The added fat produces inflammatory chemicals, creating a vicious fatigue cycle.
How to Fix It
- Women: Hormone replacement therapy (HRT) started within 10 years of menopause and before age 60 can safely improve sleep quality and reduce hot flashes. Consult your doctor about risks and benefits.
- Men: Testosterone replacement therapy can reduce fatigue but requires confirmed low levels on two morning blood tests and medical evaluation to rule out other causes.
Learn more about hormone therapy timing and safety from this clinical review.
3. Chronic Inflammation: The Silent Energy Thief
Think of chronic inflammation as a slow-burning fire inside your body that drains your energy without your awareness. This process, called inflammaging, is linked directly to chronic fatigue.
When your immune system senses inflammation, it triggers “sickness behavior”—a survival mechanism that makes you feel tired and withdrawn to conserve energy. But with chronic inflammation, your brain stays stuck in this mode.
One source of this inflammation is senescent cells—aging cells that stop functioning properly but release inflammatory chemicals, disrupting your body’s productivity. Another is your gut, where age-related changes in bacteria and a weakened barrier allow toxins to leak into the bloodstream, further fueling inflammation.
Visceral fat (fat around your organs) acts like an inflammatory organ itself, releasing molecules like interleukin-6 (IL-6) and TNF-alpha that cross into your brain and keep the fatigue signals going.
You can measure this inflammation with blood tests like high sensitivity C-reactive protein (hs-CRP) and IL-6 levels, which correlate strongly with how tired you feel.
How to Fix It
- Adopt an Anti-Inflammatory Diet: The Mediterranean diet—rich in omega-3 fatty acids, colorful vegetables, polyphenols, and fiber—is well-studied for reducing inflammation and supporting gut health.
- Exercise Regularly: Physical activity reduces visceral fat and releases anti-inflammatory molecules called myokines.
This combination lowers inflammatory markers and can break the fatigue-inflammation cycle. For a deeper dive, see this study on inflammaging and fatigue.
4. Nutritional Deficiencies: Energy Production Bottlenecks
While I didn’t dive deeply into specific nutrients in the video, it’s important to note that deficiencies in key vitamins and minerals can create bottlenecks in your body’s energy production systems. For example:
- Iron deficiency can cause anemia and fatigue.
- Vitamin D deficiency is linked to low energy and muscle weakness.
- B vitamins are critical for mitochondrial function and energy metabolism.
Getting routine blood work and working with your healthcare provider to identify and correct nutrient gaps can make a big difference in your energy levels.
5. Unmanaged Stress: The Match That Fuels Every Fire
Chronic stress disrupts your entire system by causing dysfunction in the hypothalamic-pituitary-adrenal (HPA) axis, which controls cortisol rhythms. Instead of the natural pattern of high cortisol in the morning and low at night, stress flips this, leading to morning fatigue and nighttime restlessness.
This stress-induced cortisol imbalance worsens sleep quality, hormonal health, and inflammation, making fatigue a self-reinforcing problem.
How to Fix It
- Identify your biggest stressors and prioritize stress management techniques like mindfulness, meditation, or therapy.
- Maintain a regular sleep schedule and incorporate relaxation routines before bed.
- Engage in regular physical activity which also helps regulate cortisol.
Putting It All Together: Where to Start
If you wake up tired despite sleeping, start by addressing your sleep quality—consider cognitive behavioral therapy for insomnia or improving your sleep environment.
If you’re a woman struggling with hot flashes or a man with persistent low energy, have your hormones checked and discuss treatment options with your healthcare provider.
If you feel chronically fatigued and “sick and tired,” focus on reducing inflammation through diet and exercise.
And don’t underestimate the role of stress—managing it effectively can help reset your entire system.
Remember, these five biological drivers—brain chemistry changes, hormonal shifts, chronic inflammation, nutrient gaps, and stress overload—are all fixable with targeted action and the right support.
For more science-backed tips on feeling better and living longer, keep exploring and take control of your health.
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