There are other things to do in bed besides sleeping.

...and then there is the population growth problem...

Will Stewart : "...and then there is the population growth problem..."

If it were an enlightened society aware of overpopulation and starting from zero, surely contraception would either be free and/or tantamount to mandatory?

That would be an enlightened society that still has manufacturing capability. We are talking about the time after there is no electric for lights requiring candles or early bedtimes. Such a society will NOT be manufacturing contraceptives.