Smart rotation on Xperia Z3 automatically rotates the screen based on your condition and the way you have hold your device. To enable smart rotation on Xperia Z3 do as follows:
1. Open Settings and go to the Display settings
2. Tap to open the Screen rotation settings
3. Make sure the Auto-rotate screen option is marked, and then tap the Smart screen rotation option
4. Use the slider button on the top to enable smart screen rotation on Xperia Z3.
5. Now, you can lie down and the screen rotates accordingly