Only just realised why my MBA keeps going to sleep on my lap while I am working in front of the TV. My iPhone 6 in my pocket! The magnet in the case is interfering with the Hall Effect switch on the left hand side of the body of the Mac. There is a simple fix though. Took me ages to work it out and implement it properly.
Take the phone out of your pocket, and remember, you need to do this each time you sit down to work on your sofa!