Question

Although its not a curtain
This gets closed every night
So that you can go to sleep
Its what gives you your sight

What it is?

Answer 1

It's our Eyes, eye lids act like curtains and the eye as a whole is responsible for our eye sight.

The riddle seems to be too easy especially with the inclusion of the 4th line. Even the first 3 lines are very revealing but the 4th one is ruining any mystery it had going for it.

Answer 2

You close your eyelids at night