Proverbial expression warning that things aren't always favorable; don't be complacent — danger or misfortune can come at any time (colloquially 'sleep with one eye open').
最近うまくいっているからといって安心するな。月夜の晩ばかりじゃないよ。
Don't be complacent just because things have been going well lately. It's not always moonlit nights.
油断すると痛い目に遭う。月夜の晩ばかりじゃないからね。
If you let your guard down you'll get hurt. Things aren't always so easy.