This depends on the operating system and type of "sleep" you use. Standby sleep wakes with any key press or mouse movement, but Hibernation or Hybrid Sleep requires a light power key press to wake and resume and takes a little longer, but saves more power. You can change what format you use in Power Settings, Advanced Settings. Hibernation is really good for laptops to save battery power, but if you are using your laptop as a desktop replacement that's always plugged in, you can choose what's best for you.
Is there a way to set "any key" to wake up from a sleep mode? Usually when my inspiron 1318 sleeps, a short press of the power-button would wake it up.