nächtlich
nighttime
Relates to something that happens, takes place, or is typical at night.
Der nächtliche Lärm aus der Bar hat die Anwohner gestört.
The nighttime noise from the bar disturbed the residents.

nighttime / nocturnal
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nighttime
Relates to something that happens, takes place, or is typical at night.
Der nächtliche Lärm aus der Bar hat die Anwohner gestört.
The nighttime noise from the bar disturbed the residents.

The adjective 'nächtlich' (nighttime/nocturnal) is derived directly from the noun 'die Nacht' (night). The ending '-lich' is often used to form adjectives from nouns that describe belonging or a characteristic.
Die nächtliche Kühle zog durch die offenen Fenster.
The nighttime coolness drifted through the open windows.
'Nächtlich' (nighttime/nocturnal) is an adjective and describes a noun. It answers the question 'What kind of...?'. It is used to assign a characteristic, for example 'der nächtliche Himmel' (the night sky).
Ein nächtlicher Spaziergang kann sehr romantisch sein.
A nighttime walk can be very romantic.
Common nouns combined with 'nächtlich' (nighttime/nocturnal) are 'Ruhe' (quiet), 'Störung' (disturbance), 'Ausflug' (outing), 'Anruf' (call), or 'Arbeit' (work).
Die Polizei erhielt einen nächtlichen Anruf wegen Ruhestörung.
The police received a nighttime call about a noise disturbance.
at night
'nachts' and 'nächtlich' both have to do with the night, but they do not belong to the same grammatical category. 'nachts' is an adverb and says when something happens: at night or regularly at night. 'nächtlich' is an adjective and describes a noun, for example a walk, a call, or a noise. Learners may confuse the words because both refer to the same time. A practical tip: ask 'When?' → 'nachts'. Ask 'What kind of?' → 'nächtlich'.

Der nächtliche Lärm vor dem Haus hat die Kinder geweckt.
The nighttime noise in front of the house woke the children.
Zu Hause schläft sie nachts oft schlecht.
At home, she often sleeps badly at night.