Research suggests that it's when your mind is tired that you are more likely to come up with creative ideas.
This sounds strange at first, but it makes sense if you consider that often the new is an unusual combination of the old. Worn down, your mind may jump to notions it might never light upon when fresh.
The ultimate version of this is the state between sleep and wakefulness, when dreamy images and ideas mix with logical ones. If you get into the habit of taking a few minutes every morning to brainstorm and jot down the ideas that come up you may find your creativity quotients soars.
(Of course you can also get my book, Creativity Now, published by Pearson and available from Amazon or your other favorite book seller.)