Why does my iPhone die overnight?

Until recently, I’d get a day and a half out of my iPhone before needing to charge it. Lately, I leave it off the charger overnight (with seemingly plenty of juice) and by morning it has powered down. Specifically, it either won’t respond to button mashing or it’ll “blip” the screen and then won’t respond. When I set it on the charger, it powers up as if it had been shut off (showing the Apple logo), but then when the “slide to unlock” screen comes up the battery shows a partial charge. What the dilly, yo?