The characteristics of very dense, narrow upper atmospheric Na layers observed above Mauna Kea, Hawaii (19¿50'N, 155¿28'W) are described. These layers were observed on a total of 16 occasions during 30 hours of lidar measurements from Jaunary 17 to 22, 1987. The most prominent sporadic layer, which formed on the night of January 21, exhibited a peak density of 2.8¿104 cm-3 near 96 km, with a full width of about 2 km. The rapid growth and decay of the layer lasted for about 40 min. The apparent Na production and decay rates of this layer were approximately 60 cm-3 s-1. Even after the decay, the narrow layer was observed continuously for almost 8 hours. During this period the layer moved downward steadily, with a mean apparent velocity of 31 cm s-1. The most significant characteristic of the 16 sporadic layers appears to be occurrence times. The layers formed either in the late evening, between 2100 and 2330 LST, or in the early morning, between 0300 and 0600 LST. The layers of the early morning began forming within a short time span of 15 min from 0253 to 0308 LST on 3 different days. The mean time of the maximum peak density of the early morning layers occurred about 6 hours after that of the late evening layers. The mechanisms responsible for creating these layers appear to be related to diurnal tides and sporadic E layers. ¿ American Geophysical Union 1988