Rules for waking a sleeping character

According to the rules under the Listen Skill in the PHB for 3.5; a sleeping character can roll a listen check with a minus 10 penalty in order to awaken, or wake up, in response to noise.
You may desire to use a different rule if the character is asleep but may wake up due to vibration (such as an earthquake).  I recommend a fortitude save.  If the character fails a DC n fortitude save, then the character wakes up.  You will need to decide the DC. 
In the sleep spell definition, it says that a sleeping character that gets wounded will wake up.