Green’s Dictionary of Slang

1. the night.

1937–84

Partridge DSUE (1984) 291/2: mid-C.18–20.

2. (UK prison) the punishment cell, solitary confinement.

1865

G.A. Sala My Diary in America II 367: In an English gaol twenty-four hours in the ‘darks’ would be a maximum of solitary confinement rarely exceeded.

3. depression.

2000

N. Griffiths Grits 399: Margaret’s so fucking far in the darks that I can hardly fucking see her. Gets like this sometimes, she does — terrible depression like.