Some applications such as Google updater will run throughout the day and night periodically, checking for software updates and the like.
If Windows 7's "Allow wake up timers" are enabled then your laptop will wake from it's slumber for any scheduled tasks such as software update checkers etc.
The Dell default power plan "Dell recommended" seems to have wake up timers enabled by default :P
Here's how to disable it:
- Control Panel -> System and Security -> Power Options
- Then click "Change plan settings" for your currently selected plan
- Click "Change advanced power settings"
- Scroll down to "Sleep" and expand the menu tree
- Expand "Allow wake timers"
- Click the word "enable" then drop-down and select "Disable" on both options if not already disabled
- Finally Click "OK"
Disabling "Allow wake timers" helped me get a better night sleep ... I hope it helps you!