The color blind among you might have noticed that the Universal Access section in Preferences allows you to invert colors on your Mac. Nocturne performs a similar task, however giving you a bit more flexibility.

Accessible from your menu bar, Nocturne lets you switch quickly between what it calls day and night vision. Even if you aren't color blind, you'll notice that night vision is much easier on the eyes when using Nocturne in a low light environment.

Nocturne's preferences can be played around with to customize day and night vision according to your needs. You can disable shadows and your desktop and create your own night vision by selecting two tints from the full color spectrum. You can also leave it in plain black and white.

Nocturne also lets you invert hue and screen. Give it a try to see what feels best for you. I felt the brightness adjustment slider was a bit limited though and couldn't really find a significant difference either way I set it.

Nocturne is a bit more advanced than the color inversion features found in Universal Access.