This is the 19443rd most frequent Korean word.
야간이라
"야간이라" is not a standalone word but could mean "because it is nighttime" depending on context.
The word "야간이라" in this sentence indicates the reason (because it is nighttime).
야간이라 사람이 적어서 편안합니다.
Since it is nighttime, there are fewer people, so it is comfortable.
Here, "야간이라" explains the cause of the quietness (due to it being nighttime).
야간이라 주변이 조용해졌습니다.
Since it is nighttime, the surroundings have become quiet.
"야간이라" suggests a possible cause or correlation (it is nighttime).