A new update to Google Maps iOS App makes it easier to use in dimly lit conditions, like at night. This is a safety feature to reduce the user’s risk when driving. This is designed to lessen the bright light from the display when switching between the app & the road. You can read more on this update here: Google Maps’ “Night Mode” Feature Makes It Easier To Navigate In The Dark
This new function is only available in the turn by turn mode, and is totally automatic.