Macbook Air to sleep also makes secondary display sleep.
OS: 10.10.3
I connect my turned-on but asleep (screen closed) Macbook Air (13-inch, Mid 2013) to an external Dell monitor using an Apple DVI to Thunderbolt adapter. After a few key presses to "wake" the MBAir the secondary display wakes and displays my desktop and login query. I login and then shortly thereafter the screen goes dark and I cannot wake the MBAir again. I have to disconnect the secondary display and eventually the MBAir will wake so I can use its screen. I have checked the arrangements of displays in System Preferences/Displays/Arrangement and it shows one screen on top of the other.
I am at a loss as to what is going on here. Any advice would be appreciated. Thank you.