You can't shut down your body like you can your laptop. If you've experienced rough nights when your body is rife with exhaustion yet sleep is elusive, that may be because you hoped merely climbing into bed would silence your racing mind.
The National Heart, Lung, and Blood Institute recommends carving out an hour of 'quiet time' before bed. You might spend it reading, meditating, listening to calming music, or taking a hot shower or bath-anything that relaxes you and doesn't involve bright artificial light.