In sleep research the measurement of brain currents (electroencephalography, EEG) plays a privotal role. EEG allows determining different sleep phases defined by distinctive waveform patterns. Using EEG deep sleep (with slow frequencies) can be separated from dream sleep (rapid eye movement, REM sleep) which closely resembles wakefulness. We investigate in combined PET and EEG experiments (see Figures 1 and 2) characteristic changes of these periods during one night. For this purpose we apply the so called polysomnography which is not only recording the EEG but other biosignals like respiration, ECG and muscle activity (see Figure 3). Our special scientific focus is on effects which sleep deprivation exert on the EEG and the underlying neurochemical processes. With increased sleepiness the fraction of 4-8 Hz increases and can be considered as a measure of sleep pressure.