'it's daytime' is a correct and usable phrase in written English.
For example, you might use it in a sentence like, "I always feel more productive when it's daytime."
Scientists discovered a molecule in the brain's master clock that signals to the rest of the body that it's daytime.
Most screens, including phones, use light that makes the brain think it's daytime.
In flight, avoid or strictly limit alcohol, and use coffee or tea to persuade the body clock that it's daytime.
But when it comes in the evening or during the night, it fools the body into thinking it's daytime.
>> CNN "There's growing concern that these glowing gadgets may actually fool our brains into thinking it's daytime.
"On Letterman and Leno, it always bothers me when they go outside the studio and it's daytime.
Only in the last 20 years have we acquired a basic biological understanding of how the eye's retina tells the circadian system it is daytime.
Outside, it was daytime, and life resumed its normal course.
It was daytime shine done just right.
It was daytime and just beginning to snow.
It is daytime, and we are in a park, I think.
