Do u put it to sleep or do u shut it down completely? If in BIOS you have wake on LAN enabled, network activity can cause it to wake from sleep. If its a shutdown, the only think that can do that is power management that is enabled to wake up the system at a specific point that is very common with dell systems.
Often this can be enabled by default & the time could be on 0300, off 0300 & think thats ok as it wont boot up like you would with the heating system timer but a system cant shut itself down from the BIOS so it would just turn on & thats it.
I would check BIOS & disable advanced power management.