Why Sporadic-E is caused by dense, patchy ionization forming in the E layer, and the E layer is a daytime, sunlight-driven region. Ionization builds after sunrise, so Es openings peak during daylight hours with a broad midday peak and a second peak in the late afternoon or early evening, often lingering past sunset in summer. Overnight the E region largely recombines and disappears, so Es is unlikely in the small hours.
Watch out The overnight choices describe when the E layer has recombined; the F2 layer is what supports long-haul nighttime work on the lower HF bands, not sporadic-E.
E for Earth-lit: the E layer needs sun, so sporadic-E is a sunrise-to-sunset phenomenon, peaking in summer.