It is usually artificial light that triggers singing during the hours of darkness, but not always, as I heard a Blackbird singing in an area that was devoid of any such light only last week.
This time of year however, you will hear juvenile males practising their singing before they come into the spotlight for the first time in the spring. Usually they warble a few notes, then stop as if deeply embarrassed at their own efforts before attempting a few more notes. It can sound hilarious especially if they are hiding deep inside a hedge, but for them it's deadly serious.