February 10, 2026, 7:02 pm | Read time: 6 minutes
Are you over 40 and feel like you had significantly more energy in the past? That what was easy to handle in your 20s now feels noticeably harder? You’re not alone in your age group. However, it’s not just a simple “age-related decline” behind it. Instead, several small biological changes coincide—right when professional, mental, and emotional demands often peak. FITBOOK author Laura Pomer examines these processes more closely and has also spoken with an expert about why people often feel exhausted in their 40s.
Why Are Many People Often Exhausted at 40? The Reasons
A recent overview article in “The Conversation,” which publishes evidence-based analyses and scientifically grounded opinion pieces by researchers, addresses this phenomenon. The article describes the various age-related physiological phases of adult life, roughly divided by decades. It contrasts the 20s, marked by particularly high biological efficiency, with the 40s, which are considered the most exhausting life stage. But why exactly are the 40s so biologically demanding?
Muscle Loss Is in Full Swing
The principle of sarcopenia is likely familiar to you. The term describes the gradual loss of skeletal muscle mass, which begins in the 30s. The effects are noticed gradually—with fitness, specifically maximum aerobic capacity and muscle endurance, suddenly and noticeably declining around age 40. This was the finding of a recent study by the Karolinska Institute in Stockholm.1
In your 20s, muscle mass is particularly high—even without targeted training—and metabolically active. This reduces the energy cost of everyday movements. More muscles mean, simply put, a lower energy price. By 40, everyday movements gradually require more energy: You feel physically exhausted more quickly.
In this context, the declining muscle mass and altered muscle metabolism are among the most important factors. Overview studies show that these processes are partly related to changes in mitochondrial function.2 The mitochondria themselves deserve closer examination.
Altered Mitochondria and Cellular Energy
Mitochondria are essential cell organelles, also known as the “powerhouses of the cells.” They convert nutrients and oxygen into the universal energy carrier adenosine triphosphate (ATP) through cellular respiration. This energy production is crucial for metabolism, heart and nerve functions, and physical performance.
In your 20s, the body has a large number of highly efficient mitochondria. They produce energy with less waste and fewer inflammation-promoting byproducts. However, as you age, the efficiency of mitochondrial energy production decreases, leading to a reduced availability of ATP. A study has shown a link between reduced mitochondrial function and increased fatigue.3 Additionally, due to decreased mitochondrial efficiency, conditions like sleep deprivation or illness are harder to compensate for.
Altered Sleep Quality—Also Due to Hormones
When you sleep less in your 20s, you’re less exhausted than in your 40s. This is because sleep is deeper in younger years. The body uses the so-called “slow-wave sleep” to regenerate. Later, sleep begins to fragment, making it harder to reach the important deep sleep phase. Cumulative fatigue develops as sleep deprivation or physical and mental exhaustion accumulate over days or weeks.
Hormones play a crucial role in sleep and the body in general. In your 20s—after puberty and before menopause for women—the human hormonal system reaches its most stable, often described as ideal, phase. This is associated with a fast metabolism, high bone density, and fertility. In the 40s, however, women, in particular, begin to experience hormonal fluctuations. This life phase marks the onset of perimenopause, characterized by cycle fluctuations. Men’s hormone levels also change, with testosterone levels beginning to decline. This affects sleep quality and energy distribution throughout the day.
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Expert: “Exhaustion Is Not Just Biological”
FITBOOK spoke with sleep medicine specialist Dr. Michael Feld, who has been studying sleep architecture and age-related changes in energy balance for many years. He confirms that the decline in biological performance beginning in the 40s contributes to the growing sense of exhaustion. However, only partially.
At least half of the perceived fatigue, according to Feld, is not biologically based but the result of specific life circumstances. He refers to the so-called “sandwich phase”: a life stage where particularly many energy-draining demands overlap. “You’re paying off a house, the kids have problems at school or are in the middle of puberty, and there are professional challenges,” Dr. Feld illustrates. You are “constantly externally controlled.”
While physical performance continues to decline later in life, many people around age 60 report a subjective increase in their energy levels. Dr. Feld also observes this phenomenon in his practice: “Objectively, older people often sleep worse—hormonally, physiologically, and often health-wise. Yet, we see significantly fewer retirees with severe distress.”
The key is not just sleep quality but the context in which it occurs. As many obligations disappear, internal tension decreases: Responsibility and time pressure lessen, sleep times become more flexible, and recovery becomes possible again. “When this phase of high involvement is over, many people also feel their exhaustion is significantly reduced,” the expert summarizes.
How to Counteract Exhaustion After 40
The 40s may be particularly demanding both biologically and socially. However, this doesn’t mean you’re helpless against exhaustion. A key lever lies within the body itself. The importance of physical activity, especially strength training, is perhaps greater at no other time in life than after 40. Just a few training sessions per week can help stabilize muscle mass and improve the efficiency of energy production in cells. Additionally, mitochondria benefit significantly from sprint interval training.
It’s equally important to consciously prioritize your own issues and responsibilities. This way, you can best reduce the exhausting influence of your current life circumstances. Incorporate breaks into your daily routine to significantly lower your subjective feeling of fatigue, and create small spaces for hobbies or social contacts. These everyday reliefs will also positively impact your sleep. Because pressure is the enemy of good sleep. With regular sleep times and overall sensible sleep hygiene, you do the rest.
As “The Conversation” summarizes, the 40s are “not an endpoint,” but rather a signal that the rules of life are changing. Those who consciously perceive this phase and adjust their actions accordingly can significantly mitigate exhaustion—making the 40s feel only half as strenuous.