The session finishes unexpectedly

Hi people, I made a system in PHP wich uses sessions. While I'm testing the
system some times the session finishes unexpectedly. Some times the system
works during one hour without problems but other times the system finishes
after five minutes and return to the login page.
I think that the session variables "forget" their values for some reason.
Something in the PHP configuration may be? But the time is not always the
same.
Well, I hope you can guide me with this problem. Ahead of time, thank you
very much.

Tom.