I've produced a web page that shows what happened to me when I conducted two nearly identical workouts while wearing a HR monitor.

The first workout shows a higher HR because it was performed on the day following a couple of LT workouts on the previous day.

The next day, I did a slightly harder workout, but it shows less stress because I began recovering from the previous hard days work even inspite the low level workout for that day.

No big deal, but these charts show that I'm one of the few people that actually works out with enough discipline and knowledge to know when and why my workouts are effective or simply a waste of time.

http://geocities.com/mercian753/excel/hrtr.html

The first page explains the workouts, the second contain the charts of heart rate and load.