How can I monitor/record IO wait time?

Trying to get a closer look at some systems that crunch a lot of data in the AM hours. Looking for bottlenecks. What can I use to monitor CPU wait time? If I wanted to sit up between 4 and 6am to watch "top" - I would... but I'd rather not do that. 

Looking for a utility like MRTG/PRTG/Orion, etc that can give me this information. 

What do you guys use?