Rest and stress - What happens in the dog's body? [Part 2]

We often see dogs lying seemingly motionless on their blanket — and yet they do not appear truly rested. Their breathing is shallow, their ears keep moving, their gaze is fixed on the window. Outwardly calm, inwardly highly active. This is exactly where the subject of this second part begins: what stress actually triggers in the body, why rest is a physiological state and not simply a cue, and why “quiet dogs” are often the ones who most urgently need help.

While in Part 1 we classified the basics of rest and activity, and in Part 3 we will look at causes and triggers, this part is about physiology: the HPA axis, cortisol, heart rate, muscle tone — and why chronic activation changes the entire organism.

Stress is measurable — and that is good news

In dogs, stress is not a vague feeling, but a physiological state with tangible markers: cortisol in blood and saliva, heart rate and heart rate variability, respiratory rate, muscle tone, pupil size, skin conductance. The pioneering work on this was carried out by Beerda and colleagues in the late 1990s. They showed that stress responses in dogs can be represented across several levels at the same time — based on behavior and physiology.

This has an important consequence for us as dog owners: We no longer have to guess whether a dog is stressed. We can learn to read the signs in the body — even when the behavior appears unremarkable. This is precisely the key to recognizing quiet stress dogs in the first place.

What happens in the body when a stimulus appears

As soon as the brain classifies a stimulus as potentially significant — a sound, another dog, a click in the door lock — two systems start running in parallel. First, the sympathetic nervous system: within fractions of a second, adrenaline and noradrenaline are released from the adrenal medulla, heart rate and blood pressure rise, muscles receive more blood flow, and the pupils dilate. This is the fast track: fight or flight.

At the same time, the HPA axis is activated — hypothalamus, pituitary gland, adrenal cortex. The hypothalamus releases CRH, the pituitary gland responds with ACTH, and the adrenal cortex then produces cortisol. This hormonal response is slower, but lasts longer. Cortisol mobilizes energy, temporarily dampens inflammation, and affects almost every organ system.

Acute versus chronic — the crucial difference

Acute stress is not a problem. On the contrary: it is adaptive. A dog who startles back from a car, scents a deer, or briefly flinches at thunder is using exactly this system. It switches on, performs its function, and ideally switches off again. This is followed by a recovery phase in which parasympathetic activity — the “rest-and-digest mode” — takes over again.

It only becomes problematic when the system no longer powers down cleanly. When stimuli come too close together, recovery is too short, or the dog has learned to remain in a state of heightened readiness. Robert Sapolsky describes this state in his widely cited book Why Zebras Don't Get Ulcers: wild animals experience acute stress in short, intense phases — whereas domesticated animals and humans often experience it chronically, because the stressors are psychological in nature and cannot simply be run away from.

Cortisol half-life: why recovery takes time

One point we explain again and again in consultations: cortisol does not disappear as soon as the stimulus is gone. The half-life in dogs is roughly around one hour — this means that after 60 minutes, half of the released cortisol is still active; after two hours, a quarter remains; and a complete return to baseline values can take significantly longer depending on the intensity of the stimulus.

In concrete terms, this means: if a dog had a conflict with another dog on the morning walk, he is not “long over it” by midday. His body is still working. And if the next stressor appears in the afternoon, the next wave begins before the first one has subsided. This is how many small peaks become a permanently elevated baseline level.

Allostatic load: when the body pays the price

Bruce McEwen coined the concept of allostatic load for exactly this phenomenon. Allostasis describes an organism’s ability to maintain stability through change: heart rate up when necessary, down again when the situation allows. Allostatic load refers to the cumulative wear and tear that occurs when these systems remain constantly activated.

With persistently elevated cortisol, we see measurable effects across several organ systems:

• Immune system: Cortisol acutely dampens inflammation, but chronically weakens immune defense. Dogs with chronic activation are more susceptible to infections and skin conditions.
• Memory and learning: The hippocampus, which is central to learning processes, has a particularly high number of cortisol receptors — and is sensitive to long-term strain. Learning becomes harder, generalization becomes poorer, and remembering familiar information becomes less reliable.
• Sleep: Deep sleep phases become less frequent and shorter. Dogs wake up more often, sleep more lightly, and do not reach the regenerative phases their body actually needs.
• reactivity: The stimulus threshold drops. What was still okay yesterday may tip today. Dogs appear more “thin-skinned,” react more intensely to familiar triggers, and need longer to come back down.

None of this happens overnight; it develops gradually over weeks and months. This is exactly why the connection is often recognized late in consultations: dog owners describe problems that appear “suddenly,” but in reality they are the result of a long physiological history.

Quiet stress dogs — the most risky form

There is one type of dog that particularly concerns us in practice: quiet stress dogs. On the outside, they are unremarkable. They lie calmly in cafés, walk politely on the Leash, do not bark, do not growl, do not chase. Dog owners often describe them as “easy-care” or “relaxed.” Inside, it looks different.

When you look more closely, these dogs show markers of chronic activation: increased muscle tone even when lying down, shallow and rapid breathing instead of deep abdominal breathing, a lack of deep sleep phases, panting in neutral situations, dilated pupils, high-frequency small attention movements with the ears or head.

This constellation is so problematic because it often goes unnoticed for a long time. There is no reason to intervene, no obvious symptom that alarms the dog owner. Until, at some point, something tips — sudden aggression, a breakdown in house training, a recurring gastrointestinal issue, an unusual reactivity that seems to come “out of nowhere.” It does not come out of nowhere. It comes from months of unnoticed allostatic load.

How we read stress in Vito and Amalia

In this respect, Vito and Amalia are like two different textbooks for us. With Vito, we see activation very early in his breathing — it becomes shallower and faster before anything is noticeable in his behavior. His facial expression initially remains soft, but the muscles in his forehead tighten slightly, and the tips of his ears tilt forward. If we then do not actively create relief for him, it takes him significantly longer to enter deep sleep — he may be lying down, but every movement at the door pulls him back up.

Amalia shows it differently: with her, muscle tone changes first. Her body becomes “firmer,” even while standing. Her tail carriage becomes higher and less mobile, her mouth closes more tightly, and licking over her nose increases. During sleep, we recognize strain by the absence of the typical dream phases with twitching and eye movements — she sleeps, but she does not rest.

This is exactly why observation is not something we do “on the side,” but a tool. Anyone who can read their own dog physically recognizes strain hours before behavior tips — and can intervene before the next wave begins.

Our Vitomalia conclusion

Rest is not a behavioral goal. Rest is a physiological state. As long as we measure stress only by behavior, we overlook precisely the dogs who would need the most help — the quiet ones, the ones who “function,” the ones who move inconspicuously in the background while their system has long been in a state of constant readiness.

What we take away from this part: stress is measurable — through cortisol, heart rate, breathing, and muscle tone. Acute stress responses are useful; chronic activation, on the other hand, changes the immune system, sleep, learning ability, and stimulus threshold. The cortisol half-life of around one hour explains why a stressed dog does not “just quickly” calm down. And allostatic load explains why gradual ongoing strain is the real risk factor, not the single dramatic event.

For us, this means: anyone who truly wants to help their dog needs to see the body, not just the behavior. In Part 3, we will look at which specific causes and triggers in everyday life prevent this system from powering down in the first place — and what we can do about it.