Two new time series techniques are employed to study the diurnal effect between thirty to sixty meters depth in the upper equatorial ocean. A modified spectral approach is proposed to identify the period of an unequally spaced series. Based on the exceedances of mixing activities, a new criterion is introduced to identify local nights and days. These two techniques together provide a simple way to demonstrate the existence of the diurnal cycle in deep stratified layers in the ocean. Instead of entertaining complicated models, the proposed approach suggests a simply way to identify the periodicity of an unequally spaced data set of this nature. Similar diurnal effects are detected in the analyses of a subsequent data set.