Make sure the laptop is not going into some kind of power saving mode after idling a while. If not the whole laptop, it might be powering down USB, for example, and if you have a USB wireless adapter, that could be the problem. Then once you return to the laptop it powers up fully and it's discoverable again.
That's what it sounds like, but it's an internal wireless adaptor. The laptop is awake but could it turn off the wireless adaptor?
It might. Power management schemes vary and yours may power down after a while. Check under power options/management, or even the wifi adapter under device manager to see what power management settings it may be using.
In the worst case, just disable power management completely and see what happens.