Chickens have some special sleep abilities that other animals do not share. Like us, they have Rapid Eye Movement (REM) sleep, where, like us they dream, although their REM only lasts a few seconds to minutes. They have the special ability to recover what is known as "lost sleep" where they catch up in sleep lost the previous night. They also show no ill effects from sleep deprivation, unlike us humans. For a prey animal, this is a valuable skill.
But perhaps the most unusual one is known as Unihemispheric Slow Wave Sleep (USWS), the ability to shut down one half of their brains and sleep with one eye close, and the other open. Studies have shown that when chickens or ducks sleep on a roost, the birds on the outside will keep their outer eyes open, and shut down their brains on the opposite side and close that eye. Those in the middle sleep with both closed. During the night the outer chickens turn 180 degrees, shut down the other half of their brains, and open the other eye. This gives the other half of their brains a chance to sleep.
This is why, when you go out to the coop in the morning, you find some chickens pointed in the opposite direction of the others. Not always, but often enough. Supposedly the Head Hens will sleep in the middle, but I have had at least one that always slept on the outside. She was more than a little OCD, and this behaviour just confirms it.
They also nap in the daytime, just like us humans sometimes do. If you see them sleeping in the daytime with both eyes closed, then you can be sure they are happy with the environment you have provided for them. So, seeing them awake at night is normal. Just another "chicken thing".