System activity monitoring tools

http://www.linuxquestions.org – Hi System activity monitoring tools - top, iotop, ntop, sar, collectl, etc - may be a good reference to judge the system activity when the system transitions to sleep state. But if I make the system transition to sleep state when i/o activity is zero during 15 minutes, for example, it won't sleep forever because slight i/o by daemons, etc occurs continuously even if no user i/o. So how can I judge the system activity to change the state by using those tools? Thanks in advance. (HowTos)