Stage 1 of NREM sleep is characterized by a low amplitude EEG, mixed frequency between range a and s (2 to 7Hz). EMG activity is usually higher than at other stages of sleep, but amplitude can vary widely. Stage 2 of NREM sleep is recognized by background activity and episodes of sleep spindles and K-complexes. Sleep spindles are short (12 to 14 Hz) waves that increase and decrease in amplitude to produce a spindle characteristic. Stage 3 NREM sleep is classified when slow waves or d waves (£ 2Hz) and high amplitude greater than 75mV (measured from lowest to highest wave - peak to peak) appear at 20 to 50% of the time of the day. record. Stage 4 NREM sleep is similar for EEG, EMG, and EOG from the previous stage; however, stage 4 is characterized by the presence of d waves in more than 50% of the time.
Accordingly, we can conclude that going through the NREM stages (1 to 4), the frequency of EEG waves decreases but their amplitude increases.
