Unplug before closing. I'm not sure where I read this, but this is the cause for the backpack cooking. When plugged in it goes into active sleep or something, not really sleep. When unplugged and it goes to battery mode, and activates the real sleep mode.
Something something windows something something shitty power management.
Try it for yourself and see if that makes a difference. It worked for me!
I remember at some point dell had a warning to not sleep your laptop and put it in a bag, as it can actually cook the lcd panel!