Is it possible to wake a machine from ARD?
Your network needs to be able to send WOL (wake-on-LAN) packets to the machines affected. If your network is subnetted, the problem becomes more difficult, if not impossible, without router config magic. This thread at Apple Communities explains the details more succinctly, and offers some solutions.

When I was managing 300+ Macs on two campuses more than a decade ago, it was impossible to wake any Macs from sleep reliably with ARD, so I set the Macs to spin down the hard drives when not in use and set the monitor sleep to a relatively short duration. The Macs never slept during the school day, and to save power, I used pmset commands to turn the Macs off after the close of the school day, and start up before school started on the morning. Naturally, people being who they are, some folks would power-down manually when they were done, but there was nothing I could do about it.

We now use JAMF JSS to manage our Mac fleet, which is thin on desktops -- laptops are the machine of choice. ARD has fallen by the wayside. I loved using it, but I'm no longer doing any serious Mac administration.