How can I make the PIR on an outside floodlight less sensitive?

We have a floodlight in the garden which comes on automatically when the PIR senses movement.

The PIR seems to be too sensitive. The light comes on occasionally when there is no obvious trigger in the garden. (No cat or bird etc has been detected.)

Is there a way to make the PIR less sensitive? I was thinking of covering part of the sensor with an opaque piece of plastic. Any other ideas?