Fix: Unity Rigidbody Sleep Not Waking on Collision

A pile of crates rests on the floor. You shoot one. It explodes outward; the others sit there. PhysX considers them sleeping and the explosion didn’t register as a touch.

The Symptom

Settled physics objects don’t respond to forces, explosions, or fast projectiles. They’ve been still long enough that PhysX deactivated their simulation. Once nudged, they behave normally.

What Causes This

PhysX puts Rigidbodies with kinetic energy below sleepThreshold to sleep. Sleeping bodies skip integration; they only wake on contact with an active body. An explosion that only adds force via AddExplosionForce won’t wake them because no contact happens.

The Fix

Step 1: Wake explicitly when applying area forces.

public void Explode(Vector3 center, float radius, float force)
{
    var hits = Physics.OverlapSphere(center, radius);
    foreach (var h in hits)
    {
        if (h.attachedRigidbody == null) continue;
        h.attachedRigidbody.WakeUp();
        h.attachedRigidbody.AddExplosionForce(force, center, radius);
    }
}

WakeUp() before the force ensures the body is integrating before the impulse is applied.

Step 2: Continuous collision on fast objects. Bullet Rigidbody → Collision Detection → Continuous Speculative (cheap, default-ish) or Continuous Dynamic (precise, costlier). Tunneling through sleeping targets stops.

Step 3: Lower sleepThreshold for active scenes. Project Settings → Physics → Sleep Threshold = 0.001 keeps bodies awake longer. Trade more CPU for fewer mysteriously-frozen objects.

Per-body Sleep Tuning

For specific bodies that should never sleep:

void FixedUpdate()
{
    if (rb.IsSleeping())
        rb.WakeUp();
}

Cost is one boolean per FixedUpdate. Pin only the few bodies that justify it.

Detect Sleep Programmatically

Useful for puzzle games (“is the marble settled?”):

if (rb.IsSleeping())
    AwardPoints();

Wait one or two FixedUpdates after expected motion before checking, since PhysX may not flip to sleep immediately.

Verifying

Drop a stack of objects, wait until they settle. Trigger an explosion at the base. With the fix, the stack scatters; without, only the directly-touched ones move.

Understanding the issue

Game physics is a contract between authoring (the body, mass, collision shapes you set) and the solver (how the engine integrates them per tick). Bugs at this boundary usually surface as 'the values look right but the behavior is wrong' - a sign that one side of the contract isn't honoring the other.

The specific bug described above is the kind that surfaces during integration rather than unit testing. It depends on a combination of factors: the asset configuration, the runtime state, the platform's specific behavior. In isolation, each piece looks correct; in combination, the bug emerges. This is why thorough integration testing - playing the actual game in realistic conditions - catches things that automated tests miss.

Why this happens

Bugs of this class are particularly easy to ship past internal QA because they often depend on specific runtime conditions - hardware combinations, network states, or asset configurations that QA didn't reproduce. Players hit them in the wild, file reports that are hard to repro, and the bug accumulates negative reviews while engineering tries to recreate the failure mode.

At the engine level, the behavior comes from a deliberate design decision in Unity. The engine team chose a particular trade-off - usually performance versus convenience, or generality versus specificity - and that trade-off has consequences when you push against it. Understanding the trade-off is what turns 'this bug is mysterious' into 'this bug is the expected consequence of this design'.

Verifying the fix

Verifying this fix in isolation is straightforward: reproduce the bug, apply the change, confirm the bug no longer reproduces. The harder verification is regression - did this fix introduce a new bug elsewhere? Run your standard regression suite, plus any tests that exercise the same code path with different inputs.

Reproducibility is the prerequisite for verification. If you can't reliably reproduce the bug pre-fix, you can't reliably verify it post-fix. Spend time getting a clean reproduction before you write any fix code. The fix is fast once you understand the reproduction; the reproduction is the slow part.

Variations to watch for

There's almost always a less obvious case where the same problem applies. The reported case is the one a player hit; the related cases hide because they're rarer or affect fewer players. After fixing the reported case, search the codebase for the pattern - one fix often unlocks several.

Adjacent bugs often share a root cause. After fixing the case you've found, spend an hour searching the codebase for similar patterns. What's the same call with different arguments? The same data flow with a different entity type? The same lifecycle issue in a sibling system? Each match is a candidate for the same fix, or a related fix that prevents future bugs of the same class.

In production

For shipping titles with a long support window, watch for this issue resurfacing after dependency updates. Engine upgrades, driver updates, OS releases - each one can resurface a bug class you thought you'd fixed because the underlying behavior changed slightly. Regression tests catch the obvious ones; player reports catch the rest.

When triaging a similar issue in production, prioritize gathering data over hypothesizing causes. A player report describes a symptom; what you need is a build SHA, a session timestamp, and ideally a screen recording or session replay. With those, the bug becomes tractable. Without them, you're guessing at hypothetical reproductions that may not match what the player actually hit.

Performance considerations

Performance implications matter when this bug class scales with player count or asset count. A bug that fires once per session is annoying; a bug that fires once per frame compounds. After fixing, profile the affected code path under realistic load. The fix that's correct for one entity may be too slow for ten thousand.

Diagnostic approach

The diagnostic tools available depend on your engine and platform. Use the engine's native profilers and debug overlays before reaching for external tools. The native tools have context that external tools lack - they know which subsystem owns the code, which assets are loaded, and what state the engine is in.

For Unity-specific diagnostics, the editor's profiler is the canonical starting point. Capture a representative frame with the symptom present; compare against a frame without the symptom; the diff often points directly at the cause. If the symptom is non-deterministic, capture multiple frames and look for the pattern - the cause is usually a state transition or a specific input value rather than a continuous effect.

Tooling and ecosystem

Modern engine versions ship better tooling for this kind of issue than older versions. If you're on an older release, the diagnostic step may take significantly longer because the tools you'd want don't exist yet. Sometimes the right answer is upgrading rather than fighting through limited tooling.

Within Unity, the relevant diagnostic surfaces include the standard frame debugger, memory profiler, and engine-specific debug overlays. Each one shows a different facet of what's happening. The frame debugger reveals draw call ordering and state transitions; the memory profiler shows allocation patterns; the debug overlay reveals per-system state. Bugs that resist one tool usually surrender to another - the trick is knowing which tool to reach for first.

Edge cases and pitfalls

Boundary conditions deserve specific testing attention. What happens when the input is zero, maximum, negative, or NaN? What happens at the start of a session vs hours in? What happens at the boundary between two systems handling the same data? These are where bugs hide and where regression tests are most valuable.

When writing a regression test for this fix, focus on the boundary conditions that surfaced the original bug. Tests that exercise the happy path catch obvious regressions; tests that exercise the boundary catch the subtler regressions that look like new bugs but are really the original returning. The latter are the tests that earn their keep over the long life of the project.

Team communication

When this bug class affects multiple teams (often the case for cross-system issues), early communication prevents duplicate work. The team that owns the symptom may not own the cause. A 15-minute conversation at the start of triage often saves hours of independent investigation.

If this fix touches a system several engineers work in, a short writeup in the team's engineering channel helps. Not a full design doc - a paragraph explaining what was wrong, what's fixed, and what to watch for. Future engineers encountering similar symptoms will search for the fix; making it findable is a small investment that pays back later.

“Wake before applying area force. Continuous collision for fast hits. Sleep threshold tuned for the scene.”

Related Issues

For Rigidbody falling through floor, see falling through floor. For trigger exit on disable, see trigger exit.

WakeUp. Continuous. The pile scatters.