When you watch a film and then search for it afterwards and "explained" comes up you know you're probably justified in not having a clue what you just watched, and when those webpages aren't in agreement on what has happened you know the film makers have literally lost the plot.
A family are hiding out in woods following an outbreak of some sort of virus. When a dad from another family stumbles into the house and then asks to bring his family in, we then around 90 minutes of paranoia around whether they have the virus or not.
The son of the first family seems to suffer from insomnia and eavesdrops on the other family, what does he discover and how does it play out.
It's all confusing, and the pay off doesn't come and given the title of the film you don't even find out what "it" is as nothing seems to come at night...unless I and the explainers have all missed something.