Look at the Auto Power On settings (Auto Power On Time) available under the Power Options in the BIOS. That is where it is at in the XPS 8910 so I assume something similar in the XPS 8900.
Start the computer and tap the F2 key to enter the bios. Once in the bios you can navigate to the Power Management Section then to Auto On Time. This is the section to set the computer to turn on at a set time of your choosing. Save the changes and Exit.