early in the morning, the air is cooler & so it is more dense, so the motor can be run slightly leaner, as the day warms up, your mixture may need to be richened, a couple of clicks, to allow for the air becoming less dense.
This is precisely backwards. I'm certain physics doesn't work backwards south of the equator.
Cooler air is more dense. Therefore, your engine is drawing more air in cooler air. More air to burn with the fuel means the mixture is LEAN. Therefore in cooler air, you need to run at a richer needle setting to prevent your engine from overheating. If the day warms up significantly (20-30 degrees F), you may want to recheck the high speed needle, leaning it out a couple clicks.
When I'm flying in winter, (30 degrees F) there is between 1/4 and 1/2 turn difference in the high speed needle setting.
Brad