Is it the OS or the app?
Check your energy saver preference, check the crash log to see what process is running.
Also if you are connected to a network, or have Airport installed network activity may be set to a high priority.
File sharers are constantanly probing ISP IP addresses insearch of compatible host.
OSX also attempts to clean up caches, and other housecleaning. I leave my machine on 24/7, it's setup to do housecleaning and backup at 2:00 am
Not at my machine now, check the Apple knowledgebase for where to change the settings.