The Omer's nighttime character is derived from the counting — "when the sickle is first put to the standing grain" (Devarim 16:9) begins the 49-day count, which starts at night. Since the counting begins at night, the reaping that initiates it must also begin at night. This is why, in the previous Movement (mishnah 5), the mishnah ruled that reaping at night is the preferred mitzvah — and reaping by day is only valid after the fact. This mishnah now places that ruling inside a universal legal framework: the Omer is nighttime; all nighttime mitzvos are valid the entire night.