Does darkness matter?
An adult who wakes up too early can look at the clock, realize there is more time to sleep and continue right into the next sleep cycle. But when a child wakes up and sees even the tiniest bit of light, his brain tells his body that it's time to wake up and start the day.
Darkness is an indicator to the brain that it’s time to sleep. Because light inhibits melatonin production and can interfere with the body's natural circadian rhythm, many babies wake up early simply from having too much light in their rooms. To prevent early morning wake-ups, I recommend using blackout curtains, regardless of the time of day.
It takes time for your baby to adjust to a new schedule, so give the blackout blinds a few weeks to work their magic.