As I understand it the phone defaults to something like 11:00 pm till 6:00 am. If you use the settings to make it all day it will just apply the sleep mode all day or to any hours you choose.
If you go into the menu>settings>power you will see the power mode. This is what I believe is being controlled. If you want to conserve your battery there are several settings you can use to optimize that for your personal use preferences. The sleep mode just modifies certain obvious ones.
Others are screen timeout, volume levels, auto updates, screen brightness etc.
If you play around with the settings menus you will find a whole lot of options that can be personalized. Have fun.