I say, and with God's success: The event in reality is the exit of something from one of the two passages, but it is hidden here, and sleep is a cause for the relaxation of the joints, so nothing is usually exposed as an exit. What is usually established is like what is certain, as a precaution in the matter of worship. Therefore, we have applied the ruling on it for ease, and we have called it an event; naming the thing after its cause, so sleep becomes a legal event. Refer to: Umdat al-Ri'ayah 1: 76, and Tabyeen al-Haqaiq 1: 10, and Majma' al-Anhar 1: 20, and al-Ikhtiyar 1: 15, and God knows best.