Instructions
Start a Nintendo DS game. Any game is able to enter sleep mode, as long as it is a Nintendo DS game.
Fold the top screen down at any time during the game. A chime will play and sleep mode will be activated. The battery light on the NDS will blink green.
Open the Nintendo DS to exit sleep mode and resume your game. A chime will play, and your game will pick-up, exactly where you left off.