An obscure forum post pointed me in the right direction:
- Search Windows for 'Intel'
- Select 'Intel HD Graphics Control Panel'
- Click the 'Power' square button
- Click 'On Battery' tab
- Disable 'Display Power Saving Technology' (I see it as the last option)
Here's the option in Intel's HD Graphics Control Panel:
Save your eyes and turn off "Display Power Saving Technology".