In tundra biomes, the day and night will last for 20 Minecraft days, mimicking the Polar Night and Midnight Sun. During the midnight sun, the sun never ever sets below the horizon, During the Polar Night, the sun will never get above the horizon. Light levels always remain at night levels, though you will get twilight outside the tundra biome. Tundra biomes are meant to be polar, I mean, it happens irl.
It is harder to grow crops in the dark season of the tundra, use torches and redstone lighting to make them grow.
Sleeping does work in a Polar Night, but it will still be dark the next day, if in the Polar Night. Sleeping will not work in the midnight sun unless it is a snowstorm.