You should turn off the light before going to sleep.
Literal
Sleep before electricity [object-を] extinguish [better-ほうがいい] [assertive-よ].
眠る前に ('before sleeping') uses 前に for 'before X.' 電気を消す ('turn off the electricity/light') — 電気 here is a metonymic extension for 'electric light.' ~たほうがいい = 'had better / should.'