Sleep mode is a standby feature that allows a platform to reserve power while still keeping the system on and the game's progress intact. Sleep mode has been present in several Nintendo handhelds starting with the Game Boy Advance up to the Nintendo 3DS. With the Nintendo DS line in addition to the Game Boy Advance SP, sleep mode can be activated simply by slowing the handheld shut, while with the GBA putting the system into sleep mode is a bit more complicated. Sleep mode is proved to be very useful while you are in the middle of a video game and you need to do something quick and you don't want to turn off your system. It also saves battery life.
Game Boy Advance
The Game Boy Advance's sleep mode causes the screen and sound to shut off while still retaining the game's progress. The purpose of this is to augment the system's battery power. Users can put their system into sleep mode by going into a game's menu and doing it manually. Getting the system out of sleep mode is a bit more complicated. To do so, the player has to press the L and R buttons at the same time and hold down select. After releasing these buttons, the system will turn back on. This also works the same way with the GBA Micro. On the Game Boy Advance SP, Nintendo DS, DS Lite, DSi, DSi XL and 3DS. You just close the systems while they are still turned on and then open them back up when you are ready to play again.
Sleep Mode Glitches
There are two sleep mode glitches with nintendo consoles caused by the magnets. It is recommended to not do these glitches though because it has messed up the magnets on some systems.
Sleep Mode Glitch #1
The first glitch is very popular on YouTube and has become an internet meme. If you put a newer DS on an older DS (for example if you put a Nintendo 3DS on a Nintendo DSi or if you put a Nintendo DS Lite on a Nintendo DS Phat) the system on top will get out of sleep mode while it is closed.
Sleep Mode Glitch #2
This is a very similliar glitch. If you put a magnet right next to a Nintendo DS touching the surface of the system while it is closed the system will turn on while it is closed or it will turn off while it is open.