User blog comment:NOBODY/April Fools - your jokes are needed!/@comment-5055855-20130330073047/@comment-4997528-20130331000535

It's Called timeout. It's to make sure that if you forget to, you get auto-logged out, so others who happen to pass by your computer can't abuse your account. (Not very effective I think)

To avoid this check the stay logged in box while logging in. You will not be logged out forever unless
 * 1) u log out
 * 2) u close your browser