Spiritual nighttime:[1]
Just as in the physical world there exists day and night, and the definition of night is a lack of daylight, similarly there exists a spiritual nighttime, which is a time when the body lacks the revelation of the G-dly soul. When a person is awake, the G-dly soul shines in his body and when the person goes to sleep, the G-dly soul ascends from the body. This is considered spiritual night. It is for this reason that nighttime is considered a time of complete sleep, as during this time when one sleeps the G-dly soul ascends from the body.
[1] Likkutei Torah Pinchas p. 79b