Rapid changes in scores on the two largest principal components of the electroencephalographic spectrum demarcate the boundaries of drowsy sleep

Neurobiological mechanisms determining a possibility of parsimonious descriptions of the continuous sleep process as a sequence of a few all-or-nothing variables called sleep stages remain unknown. We tested a suggestion that stage 1 sleep (drowsy sleep) corresponds to a rapid decay of a wake-promoting process and that the boundary with stages 2 separates this decay from a rapid buildup of a sleep-promoting process. The analyzed dataset included power spectra calculated from the electroencephalographic (EEG) records obtained during attempts of 15 adults to stay permanently awake for 43-61h and during multiple napping attempts of nine sleep-deprived, nine sleep-restricted, and 11 sleep-unrestricted adults. The time courses of scores on the 1st and 2nd principal components of the EEG spectra reflected the suggested phase relationships between rapid changes in the sleep- and wake-promoting processes, respectively. The 1st principal component score was permanently attenuated during wakefulness and stage 1 sleep but started to build up on the boundary with stage 2. In contrast, the 2nd principal component score started to fall down near the wake-sleep boundary but remained unchanged across stage 2. We concluded that stage 1 sleep corresponds to the decay phase of the wake-promoting process that precedes the buildup phase of the sleep-promoting process during stage 2.