In my model, humans have an average performance, that increases over time (unless something terrible happens) and a random component, that makes performance flu

In my model, humans have an average performance, that increases over time (unless something terrible happens) and a random component, that makes performance fluctuate around the average. The question is: is the day-to-day random variation much bigger than the day-to-day average increase? If so, regression towards the mean makes total sense.