The new Dell laptops only support "modern sleep". Linux doesn't support this very well, so while things look to go to sleep (eg, screens turn off) the system is still on, just idling. Since Linux doesn't have all the hardware support that Dell's drivers for Windows might, not all hardware devices are put into a low power mode.

So often what happens is nothing: the battery often drains completely overnight and it's guaranteed to be dead on Monday morning if I let it "sleep" on Friday.