Clamshell mode operation

I've got my Macbook Pro connected to an external monitor, bluetooth trackpad and bluetooth keyboard. I can put it in sleep mode and wake it up via the keyboard or trackpad.

If I shut it down, is there a way to boot it back up without opening the computer and depressing the on/off button?

Is there any harm in keeping it in sleep mode for an extended period of time i.e. 18-24 hours? I'm just trying to conserve energy.