I cant see a reason why you would want to servers to go to sleep in an environment like that regardless of whether or not they are virtualised . In a data centre the servers need to be to be there as and when people require the resources they provide not a few seconds later.
Actually in a VMware environment you can have VMware automatically power down the physical servers in order to save power when the hosts aren’t needed, then have them automatically power back up when more resources are needed. This is down to save power and cooling in the data center when load is low (nights, weekends, etc). With a large VMware server farm this could lead to a large cost savings over time.
With normal physical servers there wouldn’t be much use is sleeping machines as there wouldn’t be an easy way to bring them back online.