yes actually you can, and its pretty simple depending on which ds lite you have
some of the later versions have this gunky gel covering the reed switch.
Basically sleep mode is activated by the speakers which are magnetic close the reed switch so all you have to do is remove it.
It's the large black looking component right next to the power switch, between it and the connector for the wifi/bios chip, remove that, and you'll have it......omg i just had an awesome idea for mod from this
Thanks man