We have examined quantitatively the influence a low-latitude, premidnight sporadic E layer might have on the daily and hourly development of equatorial spread F (ESF). In particular, we calculated changes in the flux tube--integrated Pedersen conductivity as it affects the growth rate of the Rayleigh-Taylor instability, which governs the initial development of ESF. We find that the growth rate is lowered by an order of magnitude with a density of 1 ¿ 106 cm-3 in a slab from 115 to 120 km. Since sporadic E layers observed after dusk do not regularly reach these values, they are not a likely source of the daily variability in ESF. However, even a mild enhancement in the postsunset E region could lead to a significant suppression of ESF if it also inhibits the upward plasma drift of the prereversal enhancement, a key variable in the growth rate of the equatorial spread F instability. Thus, consistent with the nature of an instability, the second-order effect (suppressed upward drift) is more important than the first-order cause (reduced F region to E region conductivity) of inhibited ESF onset.