Energy savings: hard disk standby

I'd like to know how to arrange to have my hard disk go into standby mode when my system has been inactive for a while. Having seen several references here to the hdparm program for setting hard disk parameters, I took a look at the man page for it. It provides parameters for placing the disk into various standby modes, but it doesn't have anything for checking inactivity. If my system has been inactive for X minutes, I could use hdparm to place the disk into standby mode. But then the question is: what can I use to detect the inactivity so as to initiate the hdparm call? Paul