This chapter focuses on observations from a wide variety of oceanographic environments to show that the time variability of turbulence, and in turn, stratification, presents further problems to the modeler that only add to the burden of computation. It considers the energetics of stratifying or mixing a coastal body of water, in terms of vertical fluxes of buoyancy and turbulent kinetic energy. The various processes are treated, and quantified where possible, in order to build an equation which may be used as an indicator of the likelihood, or otherwise, of encountering stratification. The chapter also considers processes in various combinations in order to find the signature of time dependency that they introduce into the stratification signal. It identifies the dominant processes in a number of environmental observations of periodic stratification at scales from shelf seas to small estuaries or fjords, while also drawing on results from laboratory experiments. The chapter discusses the implications of periodic stratification.