As we make the transition from stage 1 to stage 2 sleep a number of changes occur in the EEG that signal the increasing depth of sleep. Two prominent markers of stage 2 sleep are K complexes and sleep spindles. K complexes are large waves that stand out from the background and often occur in response to environmental stimuli such as sounds in the bedroom. Sleep spindles are brief bursts of fast activity that appear something like the shape of an eye as they rapidly increase in amplitude and then rapidly decay.