外が明るくなってきたって英語でなんて言うの？

If you want to comment on the sky during your commute back to your house, you could say "by the time I finish my night shift, the sun is already starting to rise."

You spent the evening chatting with your travelling companion around the camp fire. You slept in the forest but were woken early in the morning by the sound of the dawn chorus and the howling wolves. You decided to make an early start and saddled up the horses. Beyond the covering of foliage you could see the first signs of daylight. You comment to your partner: "Dawn's coming." "I guess so."

when spring is getting nearer we have increased daylight hours. We can express this by saying "the days are getting longer now". or "it's brighter outside now". or "The mornings are getting lighter".

The noun 'dawn' means the light that shows just before sunrise. It is the first sign that the night is ending and that day is about to break, what is referred to as 'day break'. This happens often if one has been deeply engrossed in his/her work right through the night. One hardly notices how fast time flies. So, you may say: It appears that dawn has broken. or Its daylight already.