screen switches off

Hello all, when my laptop runs on battery, it switches off the screen after a short wile during which it doesn't receive any input. This wile is about 2 minutes and is too short for me. Any idea how I can change this? The problem with this is: when I move the mouse to turn on the screen again, the screen turns on but then neither the mouse nor the keybord responds. Only after I press the power button to turn the laptop to suspend mode and then call it back, keyboard and mouse work again. That's quite annoying. Flo