"Добрый вечер" is a greeting that is told in the evening (вечером). "Спокойной ночи" is used when you're going to bed at night (вечером или ночью). But if this happens at daybreak or at any other time of day except night, then the phrase sounds strange and it's better to say something like "Have a good sleep/nap/rest" ("Желаю хорошо выспаться/поспать/отдохнуть"). "Доброй ночи" can be used both in case of a greeting at night and in case of saying goodbye before sleep.

good evening and good night =)