Lenovo laptop keep going to sleep and wake up again?

When i stop working with my laptop it goes to sleep mode after 5 min or so (that is what i want), but then wake up again (why?) and so on.
What can cause this?
Tried to unplug all external devices, but still same thing happens.

model: T61, OS: Vista
Thank you!

Your power management settings are set wrong (for you). Open control panel, power options and set them all to never.