The nouns day, night, morning, evening are used without articles:

#1. if day and morning mean ‘light’, and night and evening mean ‘darkness’, or if they denote a certain part of the day.

Day broke and we started.

The sun had gone and night had come. (Abrahams)

Day is meant for work, night for sleep.

It was evening. The river was before them. (Dreiser)

#2. in the expressions by day, at night, from morning till

It is easier to work by day than at night.

The definite article is used when these nouns are modified by a particularizing attribute or when the situation makes them definite.

He will never forget the day when he met her.

The night was warm and beautifully still. (Voynich)

We spent the night in the forest.

The indefinite article is used when the noun is modified by a descriptive attribute.

I spent a sleepless night.

When the nouns morning and evening are modified by the adjectives early and late, no articles are used because these adjectives do not describe the morning or night, but only show the time.

It was early morning when the train pulled into the little siding. (Abrahams)