Are you interested to see the histograms of your single layers? There is a nice little tool called "Image Layer Histogram" which you can find under TOOLS --> Image Layer Histogram, or alternatively, you can press Ctrl + H.

Here you can display the different histograms of your Layers, where the X-axis refers to the various values of that layer, and the Y-axis refers to the frequency of occurrence for that value range.

Additionally, the Mean and Std. Dev. values are depicted at the bottom of the window.