The meaning of the expression; Ere slumber’s chain has bound me is?

Read the poem and answer Questions 26 to 30.

Oft in the stilly night

Ere slumber’s chain has bound me

Fond memory brings the light

of other days around me:

The smiles, the tears

of boyhood years.

The words of love then spoken;

The eyes that shone

How dimm’d and gone

The cheerful hearts now broken!

Thus in the stilly night

Ere slumber’s chain has bound me.

A. after I wake up
B. before I sleep
C. before I dream
D. since I cannot sleep

Correct Answer:

Option D = since I cannot sleep