The pattern of slow wave activity in spontaneously occurring long sleep.

The aim of the present study was to estimate the time course of slow wave activity (SWA) in naturally occurring long sleep episodes (ad lib). Sixteen male shift workers were subjected to 24 h ambulatory polysomnography in connection with an afternoon shift. The EEG was subjected to spectral analysis (FFT) as well as to traditional sleep stage scoring. SWA (0.5-4.5 Hz band, both nonREM and REM sleep) declined exponentially and reached an asymptote by the fifth or sixth sleep cycle. However, half the subjects showed a reduced SWA in the first cycle, with a subsequent recovery in the second cycle. The SWA reduction of the first cycle was associated with a reduced REM-latency and it was suggested that uncontrolled external influences of the real life settings may have affected SWA in the first cycle. It was concluded that the decline of SWA across time may deviate from an exponential shape under real life conditions.