That is very unusual behavior. Do you have home sharing turned on?
Are these devices plugged into the same outlet?
The only way for this to be the case is if you are in the middle of watching content from home sharing. If the computer goes to sleep then it will not be sharing the content anymore and there for the Apple TV will stop displaying whatever content you are viewing. This is not something that is going to change. I have an iMac I run all of my content through and I don't ever let it go to sleep. I let the screen turn off, but the computer is never asleep.
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