Redmine

I need to check autologin sign-on in my application. I get cookies, select value by key autologin and compare it with value from DB table tokens. First time it returns id of user by this value, but later(~3-5days later after server restart) in autologin cookie value responce is not value from DB, but the same size, i.e. redmine doesn't add service information to this, such as cookie "redmine_session", but it have brand new value.
Why does it happen?
thanks, Michail.