Why do people go to bed?

What are the next two letters in the following series and why?
W A T N T L I T F S _ _

*Hint: Check Puzzle Title

A girl has as many brothers as sisters, but each brother has only half as many brothers as sisters. How many brothers and sisters are there in the family?

Buy me, smell me or deliver me, I do not change.

Who am I?

It is always ahead of me yet I can never see it. What is it?

Can you find out the smallest number that can be conveyed as the sum of three squares in three unique ways?

What question can you never answer yes to?

In the image given, you can find several triangles. Can you count them all?

What Deficiency does the below rebus represent?

VIT_MIN

You need to complete the maze by entering from the entrance marked below in the figure near the yellow circle, bottom left and leaving from the exit point near the green circle, bottom middle.

Rule of Game: You can move only by exchanging green and yellow circles.