What might be causing this?

[stupidoctopus]

I've noticed over the past few weeks that during the night, the infrared in the baby monitor seems to be losing its strength. The picture I've attached is from early this morning.

When the camera is turned on in early evening, the whole cot and room beyond is visible (after dark, with just a night light on). However very quickly, the area covered by the cameras infrared will shrink to just a small spotlight in the centre.
Why would this be happening?

[cdtaylornats]

Everything has cooled down. All infrared is is heat sensing, so as the room cools to the ambient temperature - possibly your heating goes off at night - you child is the hottest thing in the room.

[stupidoctopus]

I don't think it's that. The room has been colder and I've never seen the infrared that bad quality.
Last night, within an hour of switching it on, the infrared was only covering a small part of the cot. I fiddled about with the plug and wire and cleaned the camera lens and infrared sensor. I also adjusted the camera slithery. Whatever I did, it seems to have solved the problem for now as it managed to go the whole night without fading out.