when i always leave the 3ds in sleep mode, i lower the volume to the lowest and turn the 3d or wifi off. It saves more battery that way. You can also lower the brightness but Im too lazy to do that everytime I put it in sleep mode. Sometimes, I put my 3ds charging while in sleep mode if I'm in my house. Most importantly, I ALWAYS charge my 3DS with the adapter, and never with the craddle. I noticed that the craddle does a poor job on charging your 3ds, and sometimes it will stop charging the 3ds randomly.