Hello and thank you for using just answer!
The dimmer switch for the gauge backlights also has a nearing on this one, if you have your dimmer switch clicked all the way to bright then it will keep the navigation in day mode regardless, if you lower the dimmer switch slightly then it will dim any time that the headlamps are on.
I turned the dashboard half way down and turned the headlights on and off but the NAV screen still remains in 'DAY' mode. I don't know if you have access to the 2010 Toyota
NAV manual, but if you do please see P32. It says 'if the screen is set to day mode with the headlight switch turned on, thuis condition is memorized in the system even with the engine turned off'
I'm not sure what 'memorized in the system' means. But it removes the DAY Mode button from the DISPLAY screen and there seems to be no other way to return to DAY/NIGHT screen mode.