People aren't quite as tall at the end of the day as they are at the beginning of it, but you're not exactly "shrinking." What's happening as the day goes on is that your weight (or more accurately, the earth's gravity) compresses the disks of your spine, thus making you just a tiny bit shorter. When you're sleeping, your body recovers and your spine stretches back out, so the next morning you're standing just a bit taller again. Is this why so many people have to adjust their rearview mirrors before driving in the morning?