Anytime you’re adjusting your sleep schedule, what you’re really adjusting is your circadian rhythm, which is heavily influenced by light exposure. As Breus explains, getting at least 15 minutes of light first thing when you wake up (natural or not), signals to your brain that this is the time you should be waking up every day. (Again, remember consistency is key.)
“I think if everybody had a sunrise alarm clock, we’d all be in better shape,” he notes, adding that if you don’t have one, turning all the lights on when you wake up works too. And on top of that, if waking up earlier is your goal, you might want to ditch the blackout curtains or eye masks so your room is not dark when you wake up (which will make it more difficult to get up in the mornings).