Periodically getting locked out of back-end (admin panel)

Question

Recently I've been having recurring issues login into the admin panel. It seems that if I do not login for 24 hours, the next time I go to login my credentials are not accepted.

I know that I am typing in my username and password correctly.

In order for me to successfully login it is necessary for me to clear the following:

*/var/session/*, */var/cache/* and */var/locks/*

What are the possible reasons for this to occur?

How can this be prevented and rectified without having to clear everything?

UPDATE I changed the Cookie Lifetime value from 24 hours to 12 hours with the same result. Multiple web-browsers will not work from multiple computers in different locations. In order to login I need to clear the session directory.

Notes: Magento 1.8.0