When in Sleep Mode, Vista only uses approximately one to two percent of the laptops battery power per hour. If the laptop sleeps for an extended period (which is determined by your sleep and hibernate settings), Vista saves any open documents to include any program information that was in use to your hard disk and will shut the system down.
By default, once the lid is closed on a laptop, the computer goes in to Sleep Mode. This setting can be changed from sleep mode to hibernate mode, completely shut down or do nothing at all.