login hook (wp_login?)

I am writing a plugin that tracks user sessions so I can get statistics on who is logged in now along with length of time of their session. I need to write a cookie upon every succesfull login attempt but I don’t know what hook to use?

I have tried wp_login but $current_user is not set so I cannot access the logged in persons ID. I’ve also tried wp_authenticate but again inappropriate.

Any ideas what hook to use?