Why? This will help conserve energy and also charge your phone. If you don't plan on answering the phone while you're sleeping or after business hours, just turn it off.
[header] How to make your cell phone battery last longer [title] Turn the phone off. [step] Only do this if it's going to be for several hours; turning a phone on/off actually uses a massive amount of power itself. This will probably be the most effective and simple way of conserving your battery's power.