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Masking

When light or darkness acts directly on behaviour without ever touching the internal clock. The animal is not consulting an internal schedule - the dark simply *releases* it, the way switching off a lamp releases a moth. The treacherous part is that masking can produce behaviour indistinguishable from clock-driven behaviour: regular, punctual, at the same hour every day - for the trivial reason that the light cycle it follows is itself regular. The way to tell is to remove the cue: a real clock keeps running and drifts on its own period, while masked behaviour falls apart.