Auto Scaling Metrics
You can enable various metrics your auto scaling instances send to Cloudwatch. You can also set the frequency these metrics are collected. When Cloudwatch detects conditions that vary from the policies you have established, it will send a message that will trigger an auto scaling event in accordance with your policies.
Click on the check box in front of the specific type or types you wish to add. You can adjust the granularity by clicking on the down arrow and selecting from the pick list. Click on "Submit" when you have made all of your choices.