I keep the light on on purpose,

Hello~ Can you help me say this naturally please?


A: Hey, why don't you turn the light off?


B: ⓐI keep the light on intentionally,
ⓑI keep the light on on purpose,
if I turn the light off cockroaches come out, you know.


A: Oh, yea, they usually come out when it darks.

Say:

They usually come out when it gets dark.

(All the phrases in bold are fine.)

:-)