I just thought you might be interested in a new app called Zen Clock by Alamax that I've helped design.
The latest version just went live in the Mac App Store yesterday. Basically Zen Clock automatically changes the colour of your desktop based on the time of day - dark at night and brighter colours during the day. You can choose from an assortment of textures and graphical overlays to customise the experience - as you can see in these examples.
It supports multiple monitors & desktops and is a great way of subliminally marking the passing of time as you work at your Mac - as well as looking rather nice.
Find out more or treat yourself to a copy here.