General terms:
* Day: This is the most common noun for the period of sunlight.
* Night: This is the most common noun for the period of darkness.
* Time: Can refer to both day and night.
* Period: Can refer to both day and night.
* Cycle: Refers to the continuous cycle of day and night.
Descriptive terms for Day:
* Dawn: The beginning of the day, when the sun first appears.
* Sunrise: The moment when the sun appears above the horizon.
* Daybreak: Another term for dawn.
* Morning: The early part of the day.
* Afternoon: The period between morning and evening.
* Evening: The time before night.
* Sunlight: The light of the sun.
Descriptive terms for Night:
* Dusk: The period when the sun is setting.
* Sunset: The moment when the sun disappears below the horizon.
* Twilight: The time between sunset and complete darkness.
* Darkness: The absence of light.
* Nightfall: The time when night begins.
* Moon: The natural satellite of the Earth that reflects light, often associated with night.
* Stars: The celestial bodies that appear as points of light in the night sky.
Metaphorical terms:
* Light: Can represent day and hope.
* Darkness: Can represent night and fear.
* Shadow: Can represent the absence of light or the presence of darkness.
* Eclipse: Can symbolize a temporary blocking of the light, either day or night.
The best noun to use will depend on the context and the specific meaning you wish to convey.