At Dell's support site the knowledge base gives a few goodies from a search for 'sleep'.
I couldn't go in depth without more info about the machine/OS/etc.
But you could!
(I used my C840 as a starting point)

Otherwise...
Update OS?
Update drivers?
Update BIOS?
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10101311 (20GB- backup empeg)
10101466 (2x60GB, Eutronix/GreenLights Blue) (Stolen!)