The nervous system was not designed for the modern world. It was designed for a world of short, intense physical threats — a predator, a fall, a fight — followed by long periods of recovery. What it cannot handle well is the thing most of us experience every day: sustained, low-level stress with no clear end point and no physical resolution. The alarm never stops. The cortisol never clears. The system drifts further and further from equilibrium.
Raise the threshold above the current temperature and watch the HVAC deactivate. Then reduce hysteresis to near zero — the actuators begin to chatter. A system with no tolerance for uncertainty oscillates forever between two bad states.
The nervous system evolved to handle threats that were physical, brief, and resolvable. A predator appears. Cortisol floods the bloodstream. Heart rate increases, digestion slows, attention narrows to a point. The predator leaves or kills you. If you survive, cortisol clears, the body returns to baseline, and you sleep. The system was not built for a meeting at nine, a performance review at eleven, a worrying email at two, and a difficult phone call at five — all of which are threats but none of which resolve.
The drift from equilibrium
Homeostasis is not a static condition. It is a dynamic one — a control system continuously making small corrections to maintain a target state against constant perturbation. Under sustained stress, corrections become larger and slower until the system cannot fully correct before the next perturbation arrives. Baseline drifts.
The body keeps the score. The body forgets nothing. — Bessel van der Kolk
Hysteresis as psychological tolerance
The hysteresis slider controls the dead band — the gap between the 'turn on' and 'turn off' thresholds. A system with no hysteresis switches the actuator on and off rapidly as the measured value oscillates around the setpoint. This is called hunting, and it is mechanically and energetically expensive. Psychologically, hysteresis is the equivalent of not treating every slightly raised voice as an existential threat.
People with trauma histories often have very low hysteresis — small deviations from safety trigger large responses. This is not weakness. It is the nervous system correctly applying a model learned in conditions where small deviations were genuinely dangerous. The problem is that the model persists in conditions where it no longer serves.
On designing for recovery
The best-engineered industrial control systems build recovery time into their architecture. They do not run at maximum capacity continuously. They leave margin. They treat headroom not as waste but as the mechanism that makes sustained operation possible. The same is true of people. What looks like inefficiency — the long walk, the weekend without a screen, the afternoon that produces nothing — is the system's recovery cycle.