Open your Google calendar, go to Settings and click on Labs.
Scroll down until you see the Hide morning and night Lab and click on Enable.
Save your Settings and it will return you to your calendar.
Once you have done this, you will see thick lines on your time bar at the left side of your calendar. These lines determine when you want to start and stop seeing times on your calendar. You can drag the line up or down to adjust what area you want to be visible. Once you have it set, click on a time inside the hidden area and it will hide like shown above (12am-8am). If you need to see this area again, just click on the hidden time box.