<prompt> Sleep spindles are seen in which stageStage 1Stage 2Stage 3Stage 4  <response>: Sleep spindles are bursts of neural oscillatory activity that are generated by interplay of the thalamic reticular nucleus TRN and other thalamic nuclei during stage 2 NREM sleep in a frequency range of 11 to 16 Hz usually 12-14 Hz with a duration of 0.5 seconds or greater usually 0.5-1.5 seconds After generation in the TRN, spindles are sustained and relayed to the coex by a thalamo-thalamic and thalamo-coical feedback loops regulated by both GABAergic and NMDA-receptor mediated glutamatergic neurotransmission. Sleep spindles have been found in all tested mammalian species and in vitro cells. Ref: guyton and hall textbook of medical physiology 12 edition page number: 822,823,824 