Why does OnTriggerStay stop firing when bodies are still?

PhysX puts stationary rigidbodies to sleep to save CPU. Sleeping bodies don't generate trigger events. Disable sleep with rigidbody.sleepThreshold = 0 or wake periodically.

Should I keep all bodies awake?

No — that defeats the optimization. Better: track presence with OnTriggerEnter/Exit and a continuous timer in your own Update. The Stay event is convenient but not needed for most logic.

Kinematic Rigidbody alternative?

Kinematic bodies don't sleep. If your trigger zone holds static-like bodies, mark them Kinematic and OnTriggerStay continues firing.

Fix: OnTriggerStay Stops Firing On Sleep

Quick answer: Track presence with Enter/Exit + your own timer. Or set rigidbody.sleepThreshold = 0. Or make the body Kinematic.

Healing zone over a sleeping body. Heal stops firing. PhysX put the body to sleep, OnTriggerStay no longer ticks for it.

The Fix

public class HealZone : MonoBehaviour {
    readonly HashSet<Health> _inside = new();

    void OnTriggerEnter(Collider c) {
        if (c.TryGetComponent(out Health h)) _inside.Add(h);
    }
    void OnTriggerExit(Collider c) {
        if (c.TryGetComponent(out Health h)) _inside.Remove(h);
    }

    void FixedUpdate() {
        foreach (var h in _inside)
            h.Heal(Time.fixedDeltaTime * 10f);
    }
}

Enter/Exit fire reliably regardless of sleep. Iterate the membership set on your own timer. Sleep stays enabled, optimization preserved.

Verifying

Body lies still in zone. Heal continues per FixedUpdate tick. With OnTriggerStay alone: heal stops once body sleeps.

“Track membership. Tick yourself. Sleep stays helpful.”

Related Issues

For BuoyancyEffector2D, see buoyancy. For CharacterController step, see step edges.

Membership set. FixedUpdate ticks. Heal flows.