© moose photos
November 3, 2025
Andrea Gartenbach
Dr. Andrea Gartenbach is a specialist in internal and functional medicine and an expert in longevity. In this three-part column, she explains how modern longevity medicine reinterprets metabolism—not as simple combustion, but as a precise interplay of light, nutrition, exercise, and rest.
Find a DoctorSearch the network for suitable specialists and contact them directly
There are people who are wide awake in the morning before the alarm rings, while others remain tired even after eight hours of sleep. Some can focus and work all day, while others constantly need replenishment: coffee, sugar, new stimuli. This rarely has anything to do with discipline or willpower.
Our metabolism actually has a sense of time, a kind of internal conductor, which dictates when energy can flow and when rest is needed. When this rhythm is out of balance, everything begins to stagnate: concentration, digestion, sleep, mood. We then feel that something is wrong, but we don't know why. In longevity medicine, this point is considered crucial. Health arises where biochemistry and timing ideally come together.
Each of our cells works in cycles. It follows an internal clock that is oriented by light, food intake, and movement. This clock determines when hormones are released, nutrients are processed, and repair mechanisms are activated. When everything runs harmoniously, the day feels like a flowing sequence: clear start, stable energy, calm evening.
But in a world of artificial light, late meals, and digital constant activity, the body loses its orientation. The internal clock runs asynchronously. Cortisol rises when we should sleep. Insulin remains active even though no meal follows. The result is diffuse symptoms that many are familiar with: restless sleep, cravings, mood swings, irritability. The "orchestra" is still playing, but the "conductor" has left.
One of the simplest and at the same time most effective strategies to recalibrate this "internal conductor" comes from the research of Valter LongoTime Restricted Eating. The principle is simple: eat within a ten- to twelve-hour window – so 12–14 hours off – and then consciously nothing more.
This break puts the body in a state of biochemical order: cells begin to recycle damaged structures, eliminate metabolic waste, and shift their energy production to fat metabolism and cellular regeneration. After just a few days, insulin levels and inflammation markers can regulate, concentration increases, and sleep improves. What matters here is not rigidity, but regularity.
Many of my patients report that their overall well-being changes just by leaving out late meals. Behind this simple measure are complex mechanisms: if nighttime insulin levels drop in time, the body can release growth hormones, activate fat burning and cell regeneration. The organism reacts amazingly quickly when it regains its natural metabolic rhythm.
Light is the strongest signal for our metabolism. The first sunlight in the morning synchronizes the internal clock and controls the cortisol increase, which wakes us up. The better alarm clock is the morning light – not the screen. Exercise has a similarly structuring effect. Just 20 minutes of light physical activity after waking up improves blood sugar regulation and stabilizes energy for the day. Equally important is getting to rest in the evening in good time. Darkness or dimmed light, no smartphone in bed, and no food after 7 p.m. make falling asleep easier. Melatonin, our nighttime regeneration hormone, can only rise when insulin has already dropped.
Cortisol is often misunderstood as a stress hormone. In truth, it is a finely tuned regulatory system that activates us in the morning and calms us in the evening. Without cortisol, we couldn't get up, think, concentrate, or perform physically. It is part of our evolutionary intelligence – a kind of biochemical timekeeper that signals the body when activity is useful and when rest begins.
A healthy cortisol curve is like a sine wave: high at the start of the day, gently falling at night. This natural daily curve is evolutionarily embedded and stabilizes almost all other hormone systems, from thyroid metabolism to glucose regulation. Cortisol only becomes problematic when this rhythm is lost. Chronic stress, late working, or constant multitasking turn the curve into a chaotic pattern – too high, too late, too long.
The body remains on high alert even when there is no acute danger. The result is fatigue, insomnia, and cravings. People who structure their daily routine with fixed meals, short breaks in between, light in the morning, and calm in the evening have a significantly more stable stress regulation and can develop more inner peace as a result. It's as if the metabolism is grateful: cortisol can go back to doing what it's meant to do – provide energy, not create stress.
Metabolism loves rhythm and repetition, not out of habit, but because biochemical processes need stability. Regular meal times, consistent sleep times, and short breaks serve as orientation points for the internal system. They prevent hormonal fluctuations, stabilize blood sugar, and harmonize cortisol regulation. Small, recurring structures create long-term energy efficiency. The body works more economically when it knows what to expect and when.
In preventive medicine today, we combine molecular diagnostics with observation and patient feedback. Biomarkers like cortisol profiles, blood sugar, or HRV reflect the body's physiological response to internal and external stresses. But equally important is self-awareness: How do I feel after waking up? When am I focused, when am I exhausted?
These questions are not trivial. They translate lab values into experience. Metabolic intelligence begins with the awareness that energy is not inexhaustible and can be shaped.
Bringing lab values in line with body signals is not self-optimization, but precise prevention – the true foundation of longevity.
In my conversations with leaders or high-performance athletes who operate under prolonged stress, this is often the turning point: recognizing that rest is not a weakness, but a strategic resource. The body follows an order that cannot be tricked. Those who respect it gain strength instead of losing it.
People who structure their daily routine with regular meals, short breaks, morning light, and evening rest develop significantly more stable stress regulation and inner calm.
Perfection is not necessary, but coherence is essential. Those who rediscover their inner rhythm experience energy as something calm and reliable. Metabolism is no longer an issue, but part of the self-evidence with which one lives.
The next part of this series will focus on the role of muscle strength and protein signals – and how structure, nutrient quality, and movement shape the language of our cells.
Morning (6–10 AM)
Activate natural light, drink water, incorporate movement. Cortisol is high, metabolism is active. Ideal time for focus and mental work.
Midday (12–2 PM)
Digestion and insulin at their peak. Now the body is ready to use nutrients efficiently.
Afternoon (4–6 PM)
Slight performance drop. A short exercise session or fresh air stabilizes energy.
Evening (7–10 PM)
Last meal before 7 PM, dimmed lights, relaxation. Cortisol decreases, melatonin increases.
Night (10 PM–6 AM)
Cell repair, growth hormones, immune activity. Sleep is biochemical regeneration.
