Your body works on a rhythm, and is guided by what is known as your circadian clock. At a certain point in this rhythm; cortisol is produced - a hormone that makes you feel awake and alert. If you drink coffee at the same time that your cortisol levels are peaking, the effects of caffeine will be greatly diminished since you're experiencing a natural jolt.
By consuming caffeine when it is not needed, your body will build a faster tolerance to it, and the buzz you get from it will greatly diminish.
http://en.ilovecoffee.jp/
Can I drink coffee yet? Calculate the best time to drink your coffee or caffeine and set a reminder for your coffee time.
App was a direct inspiration from keikun17's Can I Coffee? http://keikun17.github.io/canicoffee/ - page based from http://en.ilovecoffee.jp/posts/view/110.
Launcher was made from http://romannurik.github.io/AndroidAssetStudio/icons-launcher.html
Source available at https://github.com/omatt/canicoffee
Icon sources:
https://github.com/google/material-design-icons
http://iconmonstr.com/